Method and apparatus for video coding and decoding

ABSTRACT

The invention relates to concatenating or splicing of scalable video bitstreams. There are disclosed various methods, apparatuses and computer program products for video encoding and decoding and modifying coded video bitstreams. In some embodiments, indications are provided in the coded video bitstream to indicate a layer-wise decoding start-up process. These indications may be generated by encoders or splicers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/447,952, filed Jul. 31, 2014, which claims priority to U.S.Provisional Application No. 61/860,551, filed Jul. 31, 2013, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present application relates generally to an apparatus, a method anda computer program for video coding and decoding. More particularly,various embodiments relate to coding and decoding of scalable video.

BACKGROUND

This section is intended to provide a background or context to theinvention that is recited in the claims. The description herein mayinclude concepts that could be pursued, but are not necessarily onesthat have been previously conceived or pursued. Therefore, unlessotherwise indicated herein, what is described in this section is notprior art to the description and claims in this application and is notadmitted to be prior art by inclusion in this section.

A video coding system may comprise an encoder that transforms an inputvideo into a compressed representation suited for storage/transmissionand a decoder that can uncompress the compressed video representationback into a viewable form. The encoder may discard some information inthe original video sequence in order to represent the video in a morecompact form, for example, to enable the storage/transmission of thevideo information at a lower bitrate than otherwise might be needed.

Scalable video coding refers to a coding structure where one bitstreamcan contain multiple representations of the content at differentbitrates, resolutions, frame rates and/or other types of scalability. Ascalable bitstream may consist of a base layer providing the lowestquality video available and one or more enhancement layers that enhancethe video quality when received and decoded together with the lowerlayers. In order to improve coding efficiency for the enhancementlayers, the coded representation of that layer may depend on the lowerlayers. Each layer together with all its dependent layers is onerepresentation of the video signal at a certain spatial resolution,temporal resolution, quality level, and/or operation point of othertypes of scalability.

SUMMARY

The invention relates to concatenating or splicing of scalable videobitstreams. There are disclosed various methods, apparatuses andcomputer program products for video encoding and decoding and modifyingcoded video bitstreams. In some embodiments, indications are provided inthe coded video bitstream to indicate a layer-wise decoding start-upprocess. These indications may be generated by encoders or splicers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the presentinvention, reference is now made to the following descriptions taken inconnection with the accompanying drawings in which:

FIG. 1 shows schematically an electronic device employing someembodiments of the invention;

FIG. 2 shows schematically a user equipment suitable for employing someembodiments of the invention;

FIG. 3 further shows schematically electronic devices employingembodiments of the invention connected using wireless and/or wirednetwork connections;

FIG. 4 shows schematically an embodiment of an encoder;

FIG. 5 shows schematically an embodiment of a decoder;

FIG. 6a illustrates an example of spatial and temporal prediction of aprediction unit;

FIG. 6b illustrates another example of spatial and temporal predictionof a prediction unit;

FIG. 6c depicts an example for direct-mode motion vector inference;

FIG. 7 shows an example of a picture consisting of two tiles;

FIG. 8 depicts an example of a current block and five spatial neighborsusable as motion prediction candidates;

FIG. 9a illustrates operation of the HEVC merge mode for multiviewvideo;

FIG. 9b illustrates operation of the HEVC merge mode for multiview videoutilizing an additional reference index;

FIG. 10 depicts some examples of asymmetric stereoscopic video codingtypes.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

In the following, several embodiments of the invention will be describedin the context of one video coding arrangement. It is to be noted,however, that the invention is not limited to this particulararrangement. In fact, the different embodiments have applications widelyin any environment where improvement of 3D video coding is desired. Forexample, the invention may be applicable to video coding systems likestreaming systems, DVD players, digital television receivers, personalvideo recorders, systems and computer programs on personal computers,handheld computers and communication devices, as well as networkelements such as transcoders and cloud computing arrangements wherevideo data is handled.

In the following, several embodiments are described using the conventionof referring to (de)coding, which indicates that the embodiments mayapply to decoding and/or encoding.

The H.264/AVC standard was developed by the Joint Video Team (JVT) ofthe Video Coding Experts Group (VCEG) of the TelecommunicationsStandardization Sector of International Telecommunication Union (ITU-T)and the Moving Picture Experts Group (MPEG) of InternationalOrganisation for Standardization (ISO)/International ElectrotechnicalCommission (IEC). The H.264/AVC standard is published by both parentstandardization organizations, and it is referred to as ITU-TRecommendation H.264 and ISO/IEC International Standard 14496-10, alsoknown as MPEG-4 Part 10 Advanced Video Coding (AVC). There have beenmultiple versions of the H.264/AVC standard, each integrating newextensions or features to the specification. These extensions includeScalable Video Coding (SVC) and Multiview Video Coding (MVC).

The High Efficiency Video Coding (H.265/HEVC) standard was developed bythe Joint Collaborative Team-Video Coding (JCT-VC) of VCEG and MPEG.Currently, the H.265/HEVC standard is undergoing the final approvalballots in ISO/IEC and ITU-T. The standard will be published by bothparent standardization organizations, and it is referred to as ITU-TRecommendation H.265 and ISO/IEC International Standard 23008-2, alsoknown as MPEG-H Part 2 High Efficiency Video Coding (HEVC). There arecurrently ongoing standardization projects to develop extensions toH.265/HEVC, including scalable, multiview, three-dimensional, andfidelity range extensions.

When describing H.264/AVC and HEVC as well as in example embodiments,common notation for arithmetic operators, logical operators, relationaloperators, bit-wise operators, assignment operators, and range notatione.g. as specified in H.264/AVC or a draft HEVC may be used. Furthermore,common mathematical functions e.g. as specified in H.264/AVC or a draftHEVC may be used and a common order of precedence and execution order(from left to right or from right to left) of operators e.g. asspecified in H.264/AVC or a draft HEVC may be used.

When describing H.264/AVC and HEVC as well as in example embodiments,the following descriptors may be used to specify the parsing process ofeach syntax element.

-   -   b(8): byte having any pattern of bit string (8 bits).    -   se(v): signed integer Exp-Golomb-coded syntax element with the        left bit first.    -   u(n): unsigned integer using n bits. When n is “v” in the syntax        table, the number of bits varies in a manner dependent on the        value of other syntax elements. The parsing process for this        descriptor is specified by n next bits from the bitstream        interpreted as a binary representation of an unsigned integer        with the most significant bit written first.    -   ue(v): unsigned integer Exp-Golomb-coded syntax element with the        left bit first.

An Exp-Golomb bit string may be converted to a code number (codeNum) forexample using the following table:

Bit string codeNum 1 0 0 1 0 1 0 1 1 2 0 0 1 0 0 3 0 0 1 0 1 4 0 0 1 1 05 0 0 1 1 1 6 0 0 0 1 0 0 0 7 0 0 0 1 0 0 1 8 0 0 0 1 0 1 0 9 . . . . ..

A code number corresponding to an Exp-Golomb bit string may be convertedto se(v) for example using the following table:

codeNum syntax element value 0 0 1 1 2 −1 3 2 4 −2 5 3 6 −3 . . . . . .

When describing H.264/AVC and HEVC as well as in example embodiments,syntax structures, semantics of syntax elements, and decoding processmay be specified as follows. Syntax elements in the bitstream arerepresented in bold type. Each syntax element is described by its name(all lower case letters with underscore characters), optionally its oneor two syntax categories, and one or two descriptors for its method ofcoded representation. The decoding process behaves according to thevalue of the syntax element and to the values of previously decodedsyntax elements. When a value of a syntax element is used in the syntaxtables or the text, it appears in regular (i.e., not bold) type. In somecases the syntax tables may use the values of other variables derivedfrom syntax elements values. Such variables appear in the syntax tables,or text, named by a mixture of lower case and upper case letter andwithout any underscore characters. Variables starting with an upper caseletter are derived for the decoding of the current syntax structure andall depending syntax structures. Variables starting with an upper caseletter may be used in the decoding process for later syntax structureswithout mentioning the originating syntax structure of the variable.Variables starting with a lower case letter are only used within thecontext in which they are derived. In some cases, “mnemonic” names forsyntax element values or variable values are used interchangeably withtheir numerical values. Sometimes “mnemonic” names are used without anyassociated numerical values. The association of values and names isspecified in the text. The names are constructed from one or more groupsof letters separated by an underscore character. Each group starts withan upper case letter and may contain more upper case letters.

When describing H.264/AVC and HEVC as well as in example embodiments, asyntax structure may be specified using the following. A group ofstatements enclosed in curly brackets is a compound statement and istreated functionally as a single statement. A “while” structurespecifies a test of whether a condition is true, and if true, specifiesevaluation of a statement (or compound statement) repeatedly until thecondition is no longer true. A “do . . . while” structure specifiesevaluation of a statement once, followed by a test of whether acondition is true, and if true, specifies repeated evaluation of thestatement until the condition is no longer true. An “if . . . else”structure specifies a test of whether a condition is true, and if thecondition is true, specifies evaluation of a primary statement,otherwise, specifies evaluation of an alternative statement. The “else”part of the structure and the associated alternative statement isomitted if no alternative statement evaluation is needed. A “for”structure specifies evaluation of an initial statement, followed by atest of a condition, and if the condition is true, specifies repeatedevaluation of a primary statement followed by a subsequent statementuntil the condition is no longer true.

Some key definitions, bitstream and coding structures, and concepts ofH.264/AVC and HEVC are described in this section as an example of avideo encoder, decoder, encoding method, decoding method, and abitstream structure, wherein the embodiments may be implemented. Some ofthe key definitions, bitstream and coding structures, and concepts ofH.264/AVC are the same as in a draft HEVC standard—hence, they aredescribed below jointly. The aspects of the invention are not limited toH.264/AVC or HEVC, but rather the description is given for one possiblebasis on top of which the invention may be partly or fully realized.

Similarly to many earlier video coding standards, the bitstream syntaxand semantics as well as the decoding process for error-free bitstreamsare specified in H.264/AVC and HEVC. The encoding process is notspecified, but encoders must generate conforming bitstreams. Bitstreamand decoder conformance can be verified with the Hypothetical ReferenceDecoder (HRD). The standards contain coding tools that help in copingwith transmission errors and losses, but the use of the tools inencoding is optional and no decoding process has been specified forerroneous bitstreams.

The elementary unit for the input to an H.264/AVC or HEVC encoder andthe output of an H.264/AVC or HEVC decoder, respectively, is a picture.A picture given as an input to an encoder may also be referred to as asource picture, and a picture decoded by a decoder may be referred to asa decoded picture.

The source and decoded pictures may each be comprised of one or moresample arrays, such as one of the following sets of sample arrays:

Luma (Y) only (monochrome).

Luma and two chroma (YCbCr or YCgCo).

Green, Blue and Red (GBR, also known as RGB).

Arrays representing other unspecified monochrome or tri-stimulus colorsamplings (for example, YZX, also known as XYZ).

In the following, these arrays may be referred to as luma (or L or Y)and chroma, where the two chroma arrays may be referred to as Cb and Cr;regardless of the actual color representation method in use. The actualcolor representation method in use may be indicated e.g. in a codedbitstream e.g. using the Video Usability Information (VUI) syntax ofH.264/AVC and/or HEVC. A component may be defined as an array or asingle sample from one of the three sample arrays (luma and two chroma)or the array or a single sample of the array that compose a picture inmonochrome format.

In H.264/AVC and HEVC, a picture may either be a frame or a field. Aframe comprises a matrix of luma samples and possibly the correspondingchroma samples. A field is a set of alternate sample rows of a frame andmay be used as encoder input, when the source signal is interlaced.Chroma sample arrays may be absent (and hence monochrome sampling may bein use) or may be subsampled when compared to luma sample arrays.

A partitioning may be defined as a division of a set into subsets suchthat each element of the set is in exactly one of the subsets. A picturepartitioning may be defined as a division of a picture into smallernon-overlapping units. A block partitioning may be defined as a divisionof a block into smaller non-overlapping units, such as sub-blocks. Insome cases term block partitioning may be considered to cover multiplelevels of partitioning, for example partitioning of a picture intoslices, and partitioning of each slice into smaller units, such asmacroblocks of H.264/AVC. It is noted that the same unit, such as apicture, may have more than one partitioning. For example, a coding unitof a draft HEVC standard may be partitioned into prediction units andseparately by another quadtree into transform units.

In H.264/AVC, a macroblock is a 16×16 block of luma samples and thecorresponding blocks of chroma samples. For example, in the 4:2:0sampling pattern, a macroblock contains one 8×8 block of chroma samplesper each chroma component. In H.264/AVC, a picture is partitioned to oneor more slice groups, and a slice group contains one or more slices. InH.264/AVC, a slice consists of an integer number of macroblocks orderedconsecutively in the raster scan within a particular slice group.

During the course of HEVC standardization the terminology for example onpicture partitioning units has evolved. In the next paragraphs, somenon-limiting examples of HEVC terminology are provided.

In one draft version of the HEVC standard, pictures are divided intocoding units (CU) covering the area of the picture. A CU consists of oneor more prediction units (PU) defining the prediction process for thesamples within the CU and one or more transform units (TU) defining theprediction error coding process for the samples in the CU. Typically, aCU consists of a square block of samples with a size selectable from apredefined set of possible CU sizes. A CU with the maximum allowed sizeis typically named as LCU (largest coding unit) and the video picture isdivided into non-overlapping LCUs. An LCU can further be split into acombination of smaller CUs, e.g. by recursively splitting the LCU andresultant CUs. Each resulting CU may have at least one PU and at leastone TU associated with it. Each PU and TU can further be split intosmaller PUs and TUs in order to increase granularity of the predictionand prediction error coding processes, respectively. Each PU may haveprediction information associated with it defining what kind of aprediction is to be applied for the pixels within that PU (e.g. motionvector information for inter predicted PUs and intra predictiondirectionality information for intra predicted PUs). Similarly, each TUmay be associated with information describing the prediction errordecoding process for the samples within the TU (including e.g. DCTcoefficient information). It may be signalled at CU level whetherprediction error coding is applied or not for each CU. In the case thereis no prediction error residual associated with the CU, it can beconsidered there are no TUs for the CU. In some embodiments the PUsplitting can be realized by splitting the CU into four equal sizesquare PUs or splitting the CU into two rectangle PUs vertically orhorizontally in a symmetric or asymmetric way. The division of the imageinto CUs, and division of CUs into PUs and TUs may be signalled in thebitstream allowing the decoder to reproduce the intended structure ofthese units.

The decoder reconstructs the output video by applying prediction meanssimilar to the encoder to form a predicted representation of the pixelblocks (using the motion or spatial information created by the encoderand stored in the compressed representation) and prediction errordecoding (inverse operation of the prediction error coding recoveringthe quantized prediction error signal in spatial pixel domain). Afterapplying prediction and prediction error decoding means the decoder sumsup the prediction and prediction error signals (pixel values) to formthe output video frame. The decoder (and encoder) can also applyadditional filtering means to improve the quality of the output videobefore passing it for display and/or storing it as a predictionreference for the forthcoming frames in the video sequence.

In a draft HEVC standard, a picture can be partitioned in tiles, whichare rectangular and contain an integer number of LCUs. In a draft HEVCstandard, the partitioning to tiles forms a regular grid, where heightsand widths of tiles differ from each other by one LCU at the maximum. Ina draft HEVC, a slice consists of an integer number of CUs. The CUs arescanned in the raster scan order of LCUs within tiles or within apicture, if tiles are not in use. Within an LCU, the CUs have a specificscan order.

A basic coding unit in a HEVC working draft 5 (WD5) is a treeblock. Atreeblock is an N×N block of luma samples and two corresponding blocksof chroma samples of a picture that has three sample arrays, or an N×Nblock of samples of a monochrome picture or a picture that is codedusing three separate colour planes. A treeblock may be partitioned fordifferent coding and decoding processes. A treeblock partition is ablock of luma samples and two corresponding blocks of chroma samplesresulting from a partitioning of a treeblock for a picture that hasthree sample arrays or a block of luma samples resulting from apartitioning of a treeblock for a monochrome picture or a picture thatis coded using three separate colour planes. Each treeblock is assigneda partition signalling to identify the block sizes for intra or interprediction and for transform coding. The partitioning is a recursivequadtree partitioning. The root of the quadtree is associated with thetreeblock. The quadtree is split until a leaf is reached, which isreferred to as the coding node. The coding node is the root node of twotrees, the prediction tree and the transform tree. The prediction treespecifies the position and size of prediction blocks. The predictiontree and associated prediction data are referred to as a predictionunit. The transform tree specifies the position and size of transformblocks. The transform tree and associated transform data are referred toas a transform unit. The splitting information for luma and chroma isidentical for the prediction tree and may or may not be identical forthe transform tree. The coding node and the associated prediction andtransform units form together a coding unit.

In a HEVC WD5, pictures are divided into slices and tiles. A slice maybe a sequence of treeblocks but (when referring to a so-called finegranular slice) may also have its boundary within a treeblock at alocation where a transform unit and prediction unit coincide. Treeblockswithin a slice are coded and decoded in a raster scan order. For theprimary coded picture, the division of each picture into slices is apartitioning.

In a HEVC WD5, a tile is defined as an integer number of treeblocksco-occurring in one column and one row, ordered consecutively in theraster scan within the tile. For the primary coded picture, the divisionof each picture into tiles is a partitioning. Tiles are orderedconsecutively in the raster scan within the picture. Although a slicecontains treeblocks that are consecutive in the raster scan within atile, these treeblocks are not necessarily consecutive in the rasterscan within the picture. Slices and tiles need not contain the samesequence of treeblocks. A tile may comprise treeblocks contained in morethan one slice. Similarly, a slice may comprise treeblocks contained inseveral tiles.

A distinction between coding units and coding treeblocks may be definedfor example as follows. A slice may be defined as a sequence of one ormore coding tree units (CTU) in raster-scan order within a tile orwithin a picture if tiles are not in use. Each CTU may comprise one lumacoding treeblock (CTB) and possibly (depending on the chroma formatbeing used) two chroma CTBs. A CTU may be defined as a coding tree blockof luma samples, two corresponding coding tree blocks of chroma samplesof a picture that has three sample arrays, or a coding tree block ofsamples of a monochrome picture or a picture that is coded using threeseparate colour planes and syntax structures used to code the samples.The division of a slice into coding tree units may be regarded as apartitioning. A CTB may be defined as an N×N block of samples for somevalue of N. The division of one of the arrays that compose a picturethat has three sample arrays or of the array that compose a picture inmonochrome format or a picture that is coded using three separate colourplanes into coding tree blocks may be regarded as a partitioning. Acoding block may be defined as an N×N block of samples for some value ofN. The division of a coding tree block into coding blocks may beregarded as a partitioning.

FIG. 7 shows an example of a picture consisting of two tiles partitionedinto square coding units (solid lines) which have further beenpartitioned into rectangular prediction units (dashed lines).

In HEVC, a slice may be defined as an integer number of coding treeunits contained in one independent slice segment and all subsequentdependent slice segments (if any) that precede the next independentslice segment (if any) within the same access unit. An independent slicesegment may be defined as a slice segment for which the values of thesyntax elements of the slice segment header are not inferred from thevalues for a preceding slice segment. A dependent slice segment may bedefined as a slice segment for which the values of some syntax elementsof the slice segment header are inferred from the values for thepreceding independent slice segment in decoding order. In other words,only the independent slice segment may have a “full” slice header. Anindependent slice segment may be conveyed in one NAL unit (without otherslice segments in the same NAL unit) and likewise a dependent slicesegment may be conveyed in one NAL unit (without other slice segments inthe same NAL unit).

In H.264/AVC and HEVC, in-picture prediction may be disabled acrossslice boundaries. Thus, slices can be regarded as a way to split a codedpicture into independently decodable pieces, and slices are thereforeoften regarded as elementary units for transmission. In many cases,encoders may indicate in the bitstream which types of in-pictureprediction are turned off across slice boundaries, and the decoderoperation takes this information into account for example whenconcluding which prediction sources are available. For example, samplesfrom a neighboring macroblock or CU may be regarded as unavailable forintra prediction, if the neighboring macroblock or CU resides in adifferent slice.

A syntax element may be defined as an element of data represented in thebitstream. A syntax structure may be defined as zero or more syntaxelements present together in the bitstream in a specified order.

The elementary unit for the output of an H.264/AVC or HEVC encoder andthe input of an H.264/AVC or HEVC decoder, respectively, is a NetworkAbstraction Layer (NAL) unit. For transport over packet-orientednetworks or storage into structured files, NAL units may be encapsulatedinto packets or similar structures. A bytestream format has beenspecified in H.264/AVC and HEVC for transmission or storage environmentsthat do not provide framing structures. The bytestream format separatesNAL units from each other by attaching a start code in front of each NALunit. To avoid false detection of NAL unit boundaries, encoders run abyte-oriented start code emulation prevention algorithm, which adds anemulation prevention byte to the NAL unit payload if a start code wouldhave occurred otherwise. In order to, for example, enablestraightforward gateway operation between packet- and stream-orientedsystems, start code emulation prevention may always be performedregardless of whether the bytestream format is in use or not. A NAL unitmay be defined as a syntax structure containing an indication of thetype of data to follow and bytes containing that data in the form of anRBSP interspersed as necessary with emulation prevention bytes. A rawbyte sequence payload (RBSP) may be defined as a syntax structurecontaining an integer number of bytes that is encapsulated in a NALunit. An RBSP is either empty or has the form of a string of data bitscontaining syntax elements followed by an RBSP stop bit and followed byzero or more subsequent bits equal to 0.

NAL units consist of a header and payload. In H.264/AVC and HEVC, theNAL unit header indicates the type of the NAL unit and whether a codedslice contained in the NAL unit is a part of a reference picture or anon-reference picture.

H.264/AVC NAL unit header includes a 2-bit nal_ref_idc syntax element,which when equal to 0 indicates that a coded slice contained in the NALunit is a part of a non-reference picture and when greater than 0indicates that a coded slice contained in the NAL unit is a part of areference picture. The header for SVC and MVC NAL units may additionallycontain various indications related to the scalability and multiviewhierarchy.

In HEVC, a two-byte NAL unit header is used for all specified NAL unittypes. The NAL unit header contains one reserved bit, a six-bit NAL unittype indication, a six-bit reserved field (called nuh_layer_id) and athree-bit temporal_id_plus1 indication for temporal level. Thetemporal_id_plus1 syntax element may be regarded as a temporalidentifier for the NAL unit, and a zero-based TemporalId variable may bederived as follows: TemporalId=temporal_id_plus1−1. TemporalId equal to0 corresponds to the lowest temporal level. The value oftemporal_id_plus1 is required to be non-zero in order to avoid startcode emulation involving the two NAL unit header bytes. The bitstreamcreated by excluding all VCL NAL units having a TemporalId greater thanor equal to a selected value and including all other VCL NAL unitsremains conforming. Consequently, a picture having TemporalId equal toTID does not use any picture having a TemporalId greater than TID asinter prediction reference. A sub-layer or a temporal sub-layer may bedefined to be a temporal scalable layer of a temporal scalablebitstream, consisting of VCL NAL units with a particular value of theTemporalId variable and the associated non-VCL NAL units. Without lossof generality, in some example embodiments a variable LayerId is derivedfrom the value of nuh_layer_id for example as follows:LayerId=nuh_layer_id. In the following, LayerId, nuh_layer_id andlayer_id are used interchangeably unless otherwise indicated.

It is expected that nuh_layer_id and/or similar syntax elements in NALunit header would carry information on the scalability hierarchy. Forexample, the LayerId value nuh_layer_id and/or similar syntax elementsmay be mapped to values of variables or syntax elements describingdifferent scalability dimensions, such as quality_id or similar,dependency_id or similar, any other type of layer identifier, view orderindex or similar, view identifier, an indication whether the NAL unitconcerns depth or texture i.e. depth_flag or similar, or an identifiersimilar to priority_id of SVC indicating a valid sub-bitstreamextraction if all NAL units greater than a specific identifier value areremoved from the bitstream. nuh_layer_id and/or similar syntax elementsmay be partitioned into one or more syntax elements indicatingscalability properties. For example, a certain number of bits amongnuh_layer_id and/or similar syntax elements may be used fordependency_id or similar, while another certain number of bits amongnuh_layer_id and/or similar syntax elements may be used for quality_idor similar. Alternatively, a mapping of LayerId values or similar tovalues of variables or syntax elements describing different scalabilitydimensions may be provided for example in a Video Parameter Set, aSequence Parameter Set or another syntax structure.

NAL units can be categorized into Video Coding Layer (VCL) NAL units andnon-VCL NAL units. VCL NAL units are typically coded slice NAL units. InH.264/AVC, coded slice NAL units contain syntax elements representingone or more coded macroblocks, each of which corresponds to a block ofsamples in the uncompressed picture. In a draft HEVC standard, codedslice NAL units contain syntax elements representing one or more CU.

In H.264/AVC a coded slice NAL unit can be indicated to be a coded slicein an Instantaneous Decoding Refresh (IDR) picture or coded slice in anon-IDR picture.

In a draft HEVC standard, a coded slice NAL unit can be indicated to beone of the following types.

Name of Content of NAL unit and RBSP nal_unit_type nal_unit_type syntaxstructure 0, TRAIL_N, Coded slice segment of a non- 1 TRAIL_R TSA,non-STSA trailing picture slice_segment_layer_rbsp( ) 2, TSA_N, Codedslice segment of a TSA 3 TSA_R picture slice_segment_layer_rbsp( ) 4,STSA_N, Coded slice segment of an STSA 5 STSA_R pictureslice_layer_rbsp( ) 6, RADL_N, Coded slice segment of a RADL 7 RADL_Rpicture slice_layer_rbsp( ) 8, RASL_N, Coded slice segment of a RASL 9RASL_R, picture slice_layer_rbsp( ) 10, RSV_VCL_N10 Reserved // reservednon-RAP 12, RSV_VCL_N12 non-reference VCL NAL unit 14 RSV_VCL_N14 types11, RSV_VCL_R11 Reserved // reserved non-RAP 13, RSV_VCL_R13 referenceVCL NAL unit types 15 RSV_VCL_R15 16, BLA_W_LP Coded slice segment of aBLA 17, BLA_W_DLP picture 18 BLA_N_LP slice_segment_layer_rbsp( ) 19,IDR_W_DLP Coded slice segment of an IDR 20 IDR_N_LP pictureslice_segment_layer_rbsp( ) 21 CRA_NUT Coded slice segment of a CRApicture slice_segment_layer_rbsp( ) 22, RSV_RAP_VCL22 . . .RSV_RAP_VCL23 Reserved // reserved RAP VCL 23 NAL unit types 24 . . . 31RSV_VCL24 . . . RSV_VCL31 Reserved // reserved non-RAP VCL NAL unittypes

In a draft HEVC standard, abbreviations for picture types may be definedas follows: trailing (TRAIL) picture, Temporal Sub-layer Access (TSA),Step-wise Temporal Sub-layer Access (STSA), Random Access DecodableLeading (RADL) picture, Random Access Skipped Leading (RASL) picture,Broken Link Access (BLA) picture, Instantaneous Decoding Refresh (IDR)picture, Clean Random Access (CRA) picture.

A Random Access Point (RAP) picture, which may also or alternatively bereferred to as intra random access point (IRAP) picture, is a picturewhere each slice or slice segment has nal_unit_type in the range of 16to 23, inclusive. A RAP picture contains only intra-coded slices, andmay be a BLA picture, a CRA picture or an IDR picture. The first picturein the bitstream is a RAP picture. Provided the necessary parameter setsare available when they need to be activated, the RAP picture and allsubsequent non-RASL pictures in decoding order can be correctly decodedwithout performing the decoding process of any pictures that precede theRAP picture in decoding order. There may be pictures in a bitstream thatcontain only intra-coded slices that are not RAP pictures.

In HEVC a CRA picture may be the first picture in the bitstream indecoding order, or may appear later in the bitstream. CRA pictures inHEVC allow so-called leading pictures that follow the CRA picture indecoding order but precede it in output order. Some of the leadingpictures, so-called RASL pictures, may use pictures decoded before theCRA picture as a reference. Pictures that follow a CRA picture in bothdecoding and output order are decodable if random access is performed atthe CRA picture, and hence clean random access is achieved similarly tothe clean random access functionality of an IDR picture.

A CRA picture may have associated RADL or RASL pictures. When a CRApicture is the first picture in the bitstream in decoding order, the CRApicture is the first picture of a coded video sequence in decodingorder, and any associated RASL pictures are not output by the decoderand may not be decodable, as they may contain references to picturesthat are not present in the bitstream.

A leading picture is a picture that precedes the associated RAP picturein output order. The associated RAP picture is the previous RAP picturein decoding order (if present). A leading picture may either be a RADLpicture or a RASL picture.

All RASL pictures are leading pictures of an associated BLA or CRApicture. When the associated RAP picture is a BLA picture or is thefirst coded picture in the bitstream, the RASL picture is not output andmay not be correctly decodable, as the RASL picture may containreferences to pictures that are not present in the bitstream. However, aRASL picture can be correctly decoded if the decoding had started from aRAP picture before the associated RAP picture of the RASL picture. RASLpictures are not used as reference pictures for the decoding process ofnon-RASL pictures. When present, all RASL pictures precede, in decodingorder, all trailing pictures of the same associated RAP picture. In someearlier drafts of the HEVC standard, a RASL picture was referred to aTagged for Discard (TFD) picture.

All RADL pictures are leading pictures. RADL pictures are not used asreference pictures for the decoding process of trailing pictures of thesame associated RAP picture. When present, all RADL pictures precede, indecoding order, all trailing pictures of the same associated RAPpicture. RADL pictures do not refer to any picture preceding theassociated RAP picture in decoding order and can therefore be correctlydecoded when the decoding starts from the associated RAP picture. Insome earlier drafts of the HEVC standard, a RADL picture was referred toa Decodable Leading Picture (DLP).

Decodable leading pictures may be such that can be correctly decodedwhen the decoding is started from the CRA picture. In other words,decodable leading pictures use only the initial CRA picture orsubsequent pictures in decoding order as reference in inter prediction.Non-decodable leading pictures are such that cannot be correctly decodedwhen the decoding is started from the initial CRA picture. In otherwords, non-decodable leading pictures use pictures prior, in decodingorder, to the initial CRA picture as references in inter prediction.

When a part of a bitstream starting from a CRA picture is included inanother bitstream, the RASL pictures associated with the CRA picturemight not be correctly decodable, because some of their referencepictures might not be present in the combined bitstream. To make such asplicing operation straightforward, the NAL unit type of the CRA picturecan be changed to indicate that it is a BLA picture. The RASL picturesassociated with a BLA picture may not be correctly decodable hence arenot be output/displayed. Furthermore, the RASL pictures associated witha BLA picture may be omitted from decoding.

A BLA picture may be the first picture in the bitstream in decodingorder, or may appear later in the bitstream. Each BLA picture begins anew coded video sequence, and has similar effect on the decoding processas an IDR picture. However, a BLA picture contains syntax elements thatspecify a non-empty reference picture set. When a BLA picture hasnal_unit_type equal to BLA_W_LP, it may have associated RASL pictures,which are not output by the decoder and may not be decodable, as theymay contain references to pictures that are not present in thebitstream. When a BLA picture has nal_unit_type equal to BLA_W_LP, itmay also have associated RADL pictures, which are specified to bedecoded. When a BLA picture has nal_unit_type equal to BLA_W_DLP, itdoes not have associated RASL pictures but may have associated RADLpictures, which are specified to be decoded. BLA_W_DLP may also bereferred to as BLA_W_RADL. When a BLA picture has nal_unit_type equal toBLA_N_LP, it does not have any associated leading pictures.

An IDR picture having nal_unit_type equal to IDR_N_LP does not haveassociated leading pictures present in the bitstream. An IDR picturehaving nal_unit_type equal to IDR_W_DLP does not have associated RASLpictures present in the bitstream, but may have associated RADL picturesin the bitstream. IDR_W_DLP may also be referred to as IDR_W_RADL.

When the value of nal_unit_type is equal to TRAIL_N, TSA_N, STSA_N,RADL_N, RASL_N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decodedpicture is not used as a reference for any other picture of the sametemporal sub-layer. That is, in a draft HEVC standard, when the value ofnal_unit_type is equal to TRAIL_N, TSA_N, STSA_N, RADL_N, RASL_N,RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decoded picture is notincluded in any of RefPicSetStCurrBefore, RefPicSetStCurrAfter andRefPicSetLtCurr of any picture with the same value of TemporalId. Acoded picture with nal_unit_type equal to TRAIL_N, TSA_N, STSA_N,RADL_N, RASL_N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14 may bediscarded without affecting the decodability of other pictures with thesame value of TemporalId.

A trailing picture may be defined as a picture that follows theassociated RAP picture in output order. Any picture that is a trailingpicture does not have nal_unit_type equal to RADL_N, RADL_R, RASL_N orRASL_R. Any picture that is a leading picture may be constrained toprecede, in decoding order, all trailing pictures that are associatedwith the same RAP picture. No RASL pictures are present in the bitstreamthat are associated with a BLA picture having nal_unit_type equal toBLA_W_DLP or BLA_N_LP. No RADL pictures are present in the bitstreamthat are associated with a BLA picture having nal_unit_type equal toBLA_N_LP or that are associated with an IDR picture having nal_unit_typeequal to IDR_N_LP. Any RASL picture associated with a CRA or BLA picturemay be constrained to precede any RADL picture associated with the CRAor BLA picture in output order. Any RASL picture associated with a CRApicture may be constrained to follow, in output order, any other RAPpicture that precedes the CRA picture in decoding order.

Another means of describing picture types of a draft HEVC standard isprovided next. As illustrated in Error! Reference source not found.Error! Reference source not found. the table below, picture types can beclassified into the following groups in HEVC: a) random access point(RAP) pictures, b) leading pictures, c) sub-layer access pictures, andd) pictures that do not fall into the three mentioned groups. Thepicture types and their sub-types as described in the table below areidentified by the NAL unit type in HEVC. RAP picture types include IDRpicture, BLA picture, and CRA picture, and can further be characterizedbased on the leading pictures associated with them as indicated in thetable below.

a) Random access point pictures IDR Instantaneous without associatedleading pictures decoding refresh may have associated leading picturesBLA Broken link without associated leading pictures access may haveassociated DLP pictures but without associated TFD pictures may haveassociated DLP and TFD pictures CRA Clean random may have associatedleading pictures access b) Leading pictures DLP Decodable leadingpicture TFD Tagged for discard c) Temporal sub-layer access pictures TSATemporal sub- not used for reference in the same sub-layer layer accessmay be used for reference in the same sub-layer STSA Step-wise not usedfor reference in the same sub-layer temporal sub- may be used forreference in the same sub-layer layer access d) Picture that is not RAP,leading or temporal sub-layer access picture not used for reference inthe same sub-layer may be used for reference in the same sub-layer

CRA pictures in HEVC allow pictures that follow the CRA picture indecoding order but precede it in output order to use pictures decodedbefore the CRA picture as a reference and still allow similar cleanrandom access functionality as an IDR picture. Pictures that follow aCRA picture in both decoding and output order are decodable if randomaccess is performed at the CRA picture, and hence clean random access isachieved.

Leading pictures of a CRA picture that do not refer to any picturepreceding the CRA picture in decoding order can be correctly decodedwhen the decoding starts from the CRA picture and are therefore DLPpictures. In contrast, a TFD picture cannot be correctly decoded whendecoding starts from a CRA picture associated with the TFD picture(while the TFD picture could be correctly decoded if the decoding hadstarted from a RAP picture before the current CRA picture). Hence, TFDpictures associated with a CRA may be discarded when the decoding startsfrom the CRA picture.

When a part of a bitstream starting from a CRA picture is included inanother bitstream, the TFD pictures associated with the CRA picturecannot be decoded, because some of their reference pictures are notpresent in the combined bitstream. To make such splicing operationstraightforward, the NAL unit type of the CRA picture can be changed toindicate that it is a BLA picture. The TFD pictures associated with aBLA picture may not be correctly decodable hence should not beoutput/displayed. The TFD pictures associated with a BLA picture may beomitted from decoding.

In HEVC there are two picture types, the TSA and STSA picture types,that can be used to indicate temporal sub-layer switching points. Iftemporal sub-layers with TemporalId up to N had been decoded until theTSA or STSA picture (exclusive) and the TSA or STSA picture hasTemporalId equal to N+1, the TSA or STSA picture enables decoding of allsubsequent pictures (in decoding order) having TemporalId equal to N+1.The TSA picture type may impose restrictions on the TSA picture itselfand all pictures in the same sub-layer that follow the TSA picture indecoding order. None of these pictures is allowed to use interprediction from any picture in the same sub-layer that precedes the TSApicture in decoding order. The TSA definition may further imposerestrictions on the pictures in higher sub-layers that follow the TSApicture in decoding order. None of these pictures is allowed to refer apicture that precedes the TSA picture in decoding order if that picturebelongs to the same or higher sub-layer as the TSA picture. TSA pictureshave TemporalId greater than 0. The STSA is similar to the TSA picturebut does not impose restrictions on the pictures in higher sub-layersthat follow the STSA picture in decoding order and hence enableup-switching only onto the sub-layer where the STSA picture resides.

In HEVC, a coded video sequence (CVS) may be defined as a sequence ofaccess units that consists, in decoding order, of an IRAP access unitwith NoRaslOutputFlag equal to 1, followed by zero or more access unitsthat are not IRAP access units with NoRaslOutputFlag equal to 1,including all subsequent access units up to but not including anysubsequent access unit that is an IRAP access unit with NoRaslOutputFlagequal to 1. An IRAP access unit may be an IDR access unit, a BLA accessunit, or a CRA access unit. The value of NoRaslOutputFlag is equal to 1for each IDR access unit, each BLA access unit, and each CRA access unitthat is the first access unit in the bitstream in decoding order, is thefirst access unit that follows an end of sequence NAL unit in decodingorder, or has HandleCraAsBlaFlag equal to 1. NoRaslOutputFlag equal to 1has an impact that the RASL pictures associated with the IRAP picturefor which the NoRaslOutputFlag is set are not output by the decoder.HandleCraAsBlaFlag may be set to 1 for example by a player that seeks toa new position in a bitstream or tunes into a broadcast and startsdecoding and then starts decoding from a CRA picture.

In scalable and/or multiview video coding, at least the followingprinciples for encoding pictures and/or access units with random accessproperty may be supported.

-   -   A RAP picture within a layer may be an intra-coded picture        without inter-layer/inter-view prediction. Such a picture        enables random access capability to the layer/view it resides.    -   A RAP picture within an enhancement layer may be a picture        without inter prediction (i.e. temporal prediction) but with        inter-layer/inter-view prediction allowed. Such a picture        enables starting the decoding of the layer/view the picture        resides provided that all the reference layers/views are        available. In single-loop decoding, it may be sufficient if the        coded reference layers/views are available (which can be the        case e.g. for IDR pictures having dependency_id greater than 0        in SVC). In multi-loop decoding, it may be needed that the        reference layers/views are decoded. Such a picture may, for        example, be referred to as a stepwise layer access (STLA)        picture or an enhancement layer RAP picture.    -   An anchor access unit or a complete RAP access unit may be        defined to include only intra-coded picture(s) and STLA pictures        in all layers. In multi-loop decoding, such an access unit        enables random access to all layers/views. An example of such an        access unit is the MVC anchor access unit (among which type the        IDR access unit is a special case).    -   A stepwise RAP access unit may be defined to include a RAP        picture in the base layer but need not contain a RAP picture in        all enhancement layers. A stepwise RAP access unit enables        starting of base-layer decoding, while enhancement layer        decoding may be started when the enhancement layer contains a        RAP picture, and (in the case of multi-loop decoding) all its        reference layers/views are decoded at that point.

In a scalable extension of HEVC or any scalable extension for asingle-layer coding scheme similar to HEVC, RAP pictures may bespecified to have one or more of the following properties.

NAL unit type values of the RAP pictures with nuh_layer_id greater than0 may be used to indicate enhancement layer random access points.

An enhancement layer RAP picture may be defined as a picture thatenables starting the decoding of that enhancement layer when all itsreference layers have been decoded prior to the EL RAP picture.

Inter-layer prediction may be allowed for CRA NAL units withnuh_layer_id greater than 0, while inter prediction is disallowed.

CRA NAL units need not be aligned across layers. In other words, a CRANAL unit type can be used for all VCL NAL units with a particular valueof nuh_layer_id while another NAL unit type can be used for all VCL NALunits with another particular value of nuh_layer_id in the same accessunit.

BLA pictures have nuh_layer_id equal to 0.

IDR pictures may have nuh_layer_id greater than 0 and they may beinter-layer predicted while inter prediction is disallowed.

IDR pictures are present in an access unit either in no layers or in alllayers, i.e. an IDR nal_unit_type indicates a complete IDR access unitwhere decoding of all layers can be started.

An STLA picture (STLA_W_DLP and STLA_N_LP) may be indicated with NALunit types BLA_W_DLP and BLA_N_LP, respectively, with nuh_layer_idgreater than 0. An STLA picture may be otherwise identical to an IDRpicture with nuh_layer_id greater than 0 but needs not be aligned acrosslayers.

After a BLA picture at the base layer, the decoding of an enhancementlayer is started when the enhancement layer contains a RAP picture andthe decoding of all of its reference layers has been started.

When the decoding of an enhancement layer starts from a CRA picture, itsRASL pictures are handled similarly to RASL pictures of a BLA picture.

Layer down-switching or unintentional loss of reference pictures isidentified from missing reference pictures, in which case the decodingof the related enhancement layer continues only from the next RAPpicture on that enhancement layer.

A non-VCL NAL unit may be for example one of the following types: asequence parameter set, a picture parameter set, a supplementalenhancement information (SEI) NAL unit, an access unit delimiter, an endof sequence NAL unit, an end of stream NAL unit, or a filler data NALunit. Parameter sets may be needed for the reconstruction of decodedpictures, whereas many of the other non-VCL NAL units are not necessaryfor the reconstruction of decoded sample values.

Parameters that remain unchanged through a coded video sequence may beincluded in a sequence parameter set. In addition to the parameters thatmay be needed by the decoding process, the sequence parameter set mayoptionally contain video usability information (VUI), which includesparameters that may be important for buffering, picture output timing,rendering, and resource reservation. There are three NAL units specifiedin H.264/AVC to carry sequence parameter sets: the sequence parameterset NAL unit (having NAL unit type equal to 7) containing all the datafor H.264/AVC VCL NAL units in the sequence, the sequence parameter setextension NAL unit containing the data for auxiliary coded pictures, andthe subset sequence parameter set for MVC and SVC VCL NAL units. Thesyntax structure included in the sequence parameter set NAL unit ofH.264/AVC (having NAL unit type equal to 7) may be referred to assequence parameter set data, seq_parameter_set_data, or base SPS data.For example, profile, level, the picture size and the chroma samplingformat may be included in the base SPS data. A picture parameter setcontains such parameters that are likely to be unchanged in severalcoded pictures.

In a draft HEVC, there was also another type of a parameter set, herereferred to as an Adaptation Parameter Set (APS), which includesparameters that are likely to be unchanged in several coded slices butmay change for example for each picture or each few pictures. In a draftHEVC, the APS syntax structure includes parameters or syntax elementsrelated to quantization matrices (QM), sample adaptive offset (SAO),adaptive loop filtering (ALF), and deblocking filtering. In a draftHEVC, an APS is a NAL unit and coded without reference or predictionfrom any other NAL unit. An identifier, referred to as aps_id syntaxelement, is included in APS NAL unit, and included and used in the sliceheader to refer to a particular APS. However, APS was not included inthe final H.265/HEVC standard.

H.265/HEVC also includes another type of a parameter set, called a videoparameter set (VPS). A video parameter set RBSP may include parametersthat can be referred to by one or more sequence parameter set RBSPs.

The relationship and hierarchy between VPS, SPS, and PPS may bedescribed as follows. VPS resides one level above SPS in the parameterset hierarchy and in the context of scalability and/or 3DV. VPS mayinclude parameters that are common for all slices across all(scalability or view) layers in the entire coded video sequence. SPSincludes the parameters that are common for all slices in a particular(scalability or view) layer in the entire coded video sequence, and maybe shared by multiple (scalability or view) layers. PPS includes theparameters that are common for all slices in a particular layerrepresentation (the representation of one scalability or view layer inone access unit) and are likely to be shared by all slices in multiplelayer representations.

VPS may provide information about the dependency relationships of thelayers in a bitstream, as well as many other information that areapplicable to all slices across all (scalability or view) layers in theentire coded video sequence. In a scalable extension of HEVC, VPS mayfor example include a mapping of the LayerId value derived from the NALunit header to one or more scalability dimension values, for examplecorrespond to dependency_id, quality_id, view_id, and depth_flag for thelayer defined similarly to SVC and MVC. VPS may include profile andlevel information for one or more layers as well as the profile and/orlevel for one or more temporal sub-layers (consisting of VCL NAL unitsat and below certain TemporalId values) of a layer representation.

An example syntax of a VPS extension intended to be a part of the VPS isprovided in the following. The presented VPS extension provides thedependency relationships among other things.

De- vps_extension( ) { scriptor  while( !byte_aligned( ) )  vps_extension_byte_alignment_reserved_one_bit u(1) avc_base_layer_flag u(1)  splitting_flag u(1)  for( i = 0,NumScalabilityTypes = 0; i < 16; i++ ) {   scalability_mask[ i ] u(1)  NumScalabilityTypes += scalability_mask[ i ]  }  for( j = 0; j<NumScalabilityTypes; j++ )   dimension_id_len_minus1[ j ] u(3) vps_nuh_layer_id_present_flag u(1)  for( i = 1; i <=vps_max_layers_minus1; i++ ) {   if( vps_nuh_layer_id_present_flag )   layer_id_in_nuh[ i ] u(6)   for( j = 0; j < NumScalabilityTypes; j++)    dimension_id[ i ][ j ] u(v)  }  for( i = 1; i <=vps_num_op_sets_minus1; i ++ ) {   vps_profile_present_flag[ i ] u(1)  if( !vps_profile_present_flag[ i ] )    profile_op_ref_minus1[ i ]ue(v)   profile_tier_level( vps_profile_present_flag[ i ],    vps_max_sub_layers_minus1)  }  num_output_operation_points ue(v) for( i = 0; i < num_output_operation_points; i++ ) {  output_op_point_index[ i ] ue(v)   for( j = 0 ; j <=vps_max_nuh_reserved_zero_layer_id;   j++)    if(op_layer_id_included_flag[ op_point_index[ i ] ]    [ i ] )    output_layer_flag[ op_point_index[ i ] ][ j ] u(1)  }  for( i = 1; i<= vps_max_layers_minus1; i++ ) {   for( j = 0; j < i; j++ )   direct_dependency_flag[ i ][ j ] u(1) }

The semantics of the presented VPS extension may be specified asdescribed in the following paragraphs.

vps_extension_byte_alignment_reserved_one_bit is equal to 1 and is usedto achieve alignment of the next syntax element to a byte boundary.avc_base_layer_flag equal to 1 specifies that the base layer conforms toH.264/AVC, equal to 0 specifies that it conforms to this specification.The semantics of avc_base_layer_flag may be further specified asfollows. When avc_base_layer_flag equal to 1, in H.264/AVC conformingbase layer, after applying the H.264/AVC decoding process for referencepicture lists construction the output reference picture listsrefPicList0 and refPicList1 (when applicable) does not contain anypictures for which the TemporalId is greater than TemporalId of thecoded picture. All sub-bitstreams of the H.264/AVC conforming base layerthat can be derived using the sub-bitstream extraction process asspecified in H.264/AVC Annex G with any value for temporal_id as theinput result in a set of coded video sequences, with each coded videosequence conforming to one or more of the profiles specified inH.264/AVC Annexes A, G and H.

splitting_flag equal to 1 indicates that the bits of the nuh_layer_idsyntax element in the NAL unit header are split into n segments with alength, in bits, according to the values of thedimension_id_len_minus1[i] syntax element and that the n segments areassociated with the n scalability dimensions indicated inscalability_mask_flag[i]. When splitting_flag is equal to 1, the valueof the j-th segment of the nuh_layer_id of i-th layer shall be equal tothe value of dimension_id[i][j]. splitting_flag equal to 0 does notindicate the above constraint. When splitting_flag is equal to 1, i.e.the restriction reported in the semantics of the dimension_id[i][j]syntax element is obeyed, scalable identifiers can be derived from thenuh_layer_id syntax element in the NAL unit header by a bit masked copyas an alternative to the derivation as reported in the semantics of thedimension_id[i][j] syntax element. The respective bit mask for the i-thscalable dimension is defined by the value of thedimension_id_len_minus1[i] syntax element and dimBitOffset[i] asspecified in the semantics of dimension_id_len_minus1[j].

scalability_mask[i] equal to 1 indicates that dimension_id syntaxelements corresponding to the i-th scalability dimension are present.scalability_mask[i] equal to 0 indicates that dimension_id syntaxelements corresponding to the i-th scalability dimension are notpresent. The scalability dimensions corresponding to each value of of iin scalability_mask[i] may be specified for example to include thefollowing or any subset thereof along with other scalability dimensions.

scalability_mask Scalability ScalabilityId index dimension mapping 0multiview ViewId 1 reference index DependencyId based spatial or qualityscalability 2 depth DepthFlag 3 TextureBL TextureBLDepId based spatialor quality scalability

dimension_id_len_minus1[j] plus 1 specifies the length, in bits, of thedimension_id[i][j] syntax element. The variable dimBitOffset[j] isderived as follows. The variable dimBitOffset[0] is set to 0.dimBitOffset[j] is derived to be equal to a cumulative sum in the rangeof dimIdx from 0 to j−1, inclusive, fordimension_id_len_minus1[dimIdx]+1).

vps_nuh_layer_id_present_flag specifies whether the layer_id_in_nuh[i]syntax is present. layer_id_in_nuh[i] specifies the value of thenuh_layer_id syntax element in VCL NAL units of the i-th layer. When notpresent, the value of layer_id_in_nuh[i] is inferred to be equal to i.layer_id_in_nuh[i] is greater than layer_id_in_nuh[i−1]. The variableLayerIdInVps[layer_id_in_nuh[i]] is set equal to i.

dimension_id[i][j] specifies the identifier of the j-th scalabilitydimension type of the i-th layer. When not present, the value ofdimension_id[i][j] is inferred to be equal to 0. The number of bits usedfor the representation of dimension_id[i][j] isdimension_id_len_minus1[j]+1 bits.

dimension_id[i][j] and scalability_mask[i] may be used to derivevariables associating scalability dimension values to layers. Forexample, the variables ScalabilityId[layerIdInVps][scalabilityMaskIndex]andViewId[layerIdInNuh] may be derived as follows:

for (i = 0; i <= vps_max_layers_minus1; i++) {  for( smIdx= 0, j =0;smIdx< 16; smIdx ++ )   if( ( i != 0 ) && scalability_mask[ smIdx ] )     ScalabilityId[ i ][ smIdx ] = dimension_id[ i ][ j++ ]   else     ScalabilityId[ i ][ smIdx ] = 0  ViewId[ layer_id_in_nuh[ i ] ] =ScalabilityId[ i ][ 0 ] }

Similarly, variables DependencyId[layerIdInNuh],DepthFlag[layerIdInNuh], and TextureBLDepId[layerIdInNuh] may be derivede.g. as follows:

for (i = 0; i <= vps_max_layers_minus1; i++) {  for( smIdx= 0, j =0;smIdx< 16; smIdx ++ )   if( ( i != 0 ) && scalability_mask[ smIdx ] )     ScalabilityId[ i ][ smIdx ] = dimension_id[ i ][ j++ ]   else     ScalabilityId[ i ][ smIdx ] = 0  DependencyId[ layer_id_in_nuh[ i ]] = ScalabilityId[ i ][ 1 ]  DepthFlag[ layer_id_in_nuh[ i ] ] =ScalabilityId[ i ][ 2 ]  TextureBLDepId[ layer_id_in_nuh[ i ] ] =ScalabilityId[ i ][ 3 ] }

vps_profile_present_flag[i] equal to 1 specifies that the profile andtier information for operation point i is present in theprofile_tier_level( ) syntax structure. vps_profile_present_flag[i]equal to 0 specifies that profile and tier information for operationpoint i is not present in the profile_tier_level( ) syntax structure andis inferred.

profile_op_ref_minus1[i] indicates that the profile and tier informationfor the i-th operation point is inferred to be equal to the profile andtier information from the (profile_op_ref_minus1[i]+1)-th operationpoint.

num_output_operation_points specifies the number of operation points forwhich output layers are specified with output_op_point_index[i] andoutput_layer_flag. When not present, the value ofnum_output_operation_points is inferred to be equal to 0.

output_op_point_index[i] identifies the operation point for whichoutput_layer_flag[op_point_index[i]][j] applies to.

output_layer_flag[output_op_point_index[i]][j] equal to 1 specifies thatthe layer with nuh_layer_id equal to j is a target output layer of theoperation point identified by output_op_point_index[i].output_layer_flag[output_op_point_index[i]][j] equal to 0 specifies thatthe layer with nuh_layer_id equal to j is not a target output layer ofthe operation point identified by output_op_point_index[i].

For each operation point_index j not equal to output_op_point_index[i]for any value of i in the range 0 to num_output_operation_points−1,inclusive, let highestLayerId be the greatest value of nuh_layer_idwithin the operation point of index j. output_layer_flag[j][k] isinferred to be equal to 0 for all values of k in the range of 0 to 63,inclusive, unequal to highestLayerId.output_layer_flag[j][highestLayerId] is inferred to be equal to 1.

In other words, when an operation point is not included among thoseindicated by output_op_point_index[i], the layer with the greatest valueof nuh_layer_id within the operation point is the only target outputlayer of the operation point.

direct_dependency_flag[i][j] equal to 0 specifies that the layer withindex j is not a direct reference layer for the layer with index i.direct_dependency_flag[i][j] equal to 1 specifies that the layer withindex j may be a direct reference layer for the layer with index i. Whendirect_dependency_flag[i][j] is not present for i and j in the range of0 to vps_max_num_layers_minus1, it is inferred to be equal to 0.

The variables NumDirectRefLayers[i] and RefLayerId[i][j] are derived asfollows:

for( i = 1; i <= vps_max_layers_minus1; i++ ) for( j = 0,NumDirectRefLayers[ i ] = 0; j < i; j++ ) if( direct_dependency_flag[ i][ j ] = = 1 ) RefLayerId[ i ][ NumDirectRefLayers[ i ]++ ] =layer_id_in_nuh[ j ]

H.264/AVC and HEVC syntax allows many instances of parameter sets, andeach instance is identified with a unique identifier. In order to limitthe memory usage needed for parameter sets, the value range forparameter set identifiers has been limited. In H.264/AVC and a draftHEVC standard, each slice header includes the identifier of the pictureparameter set that is active for the decoding of the picture thatcontains the slice, and each picture parameter set contains theidentifier of the active sequence parameter set. In a draft HEVCstandard, a slice header additionally contains an APS identifier.Consequently, the transmission of picture and sequence parameter setsdoes not have to be accurately synchronized with the transmission ofslices. Instead, it is sufficient that the active sequence and pictureparameter sets are received at any moment before they are referenced,which allows transmission of parameter sets “out-of-band” using a morereliable transmission mechanism compared to the protocols used for theslice data. For example, parameter sets can be included as a parameterin the session description for Real-time Transport Protocol (RTP)sessions. If parameter sets are transmitted in-band, they can berepeated to improve error robustness.

A parameter set may be activated by a reference from a slice or fromanother active parameter set or in some cases from another syntaxstructure such as a buffering period SEI message. In the following,non-limiting examples of activation of parameter sets in a draft HEVCstandard are given.

Each adaptation parameter set RBSP is initially considered not active atthe start of the operation of the decoding process. At most oneadaptation parameter set RBSP is considered active at any given momentduring the operation of the decoding process, and the activation of anyparticular adaptation parameter set RBSP results in the deactivation ofthe previously-active adaptation parameter set RBSP (if any).

When an adaptation parameter set RBSP (with a particular value ofaps_id) is not active and it is referred to by a coded slice NAL unit(using that value of aps_id), it is activated. This adaptation parameterset RBSP is called the active adaptation parameter set RBSP until it isdeactivated by the activation of another adaptation parameter set RBSP.An adaptation parameter set RBSP, with that particular value of aps_id,is available to the decoding process prior to its activation, includedin at least one access unit with temporal_id equal to or less than thetemporal_id of the adaptation parameter set NAL unit, unless theadaptation parameter set is provided through external means.

Each picture parameter set RBSP is initially considered not active atthe start of the operation of the decoding process. At most one pictureparameter set RBSP is considered active at any given moment during theoperation of the decoding process, and the activation of any particularpicture parameter set RBSP results in the deactivation of thepreviously-active picture parameter set RBSP (if any).

When a picture parameter set RBSP (with a particular value ofpic_parameter_set_id) is not active and it is referred to by a codedslice NAL unit or coded slice data partition A NAL unit (using thatvalue of pic_parameter_set_id), it is activated. This picture parameterset RBSP is called the active picture parameter set RBSP until it isdeactivated by the activation of another picture parameter set RBSP. Apicture parameter set RBSP, with that particular value ofpic_parameter_set_id, is available to the decoding process prior to itsactivation, included in at least one access unit with temporal_id equalto or less than the temporal_id of the picture parameter set NAL unit,unless the picture parameter set is provided through external means.

Each sequence parameter set RBSP is initially considered not active atthe start of the operation of the decoding process. At most one sequenceparameter set RBSP is considered active at any given moment during theoperation of the decoding process, and the activation of any particularsequence parameter set RBSP results in the deactivation of thepreviously-active sequence parameter set RBSP (if any).

When a sequence parameter set RBSP (with a particular value ofseq_parameter_set_id) is not already active and it is referred to byactivation of a picture parameter set RBSP (using that value ofseq_parameter_set_id) or is referred to by an SEI NAL unit containing abuffering period SEI message (using that value of seq_parameter_set_id),it is activated. This sequence parameter set RBSP is called the activesequence parameter set RBSP until it is deactivated by the activation ofanother sequence parameter set RBSP. A sequence parameter set RBSP, withthat particular value of seq_parameter_set_id is available to thedecoding process prior to its activation, included in at least oneaccess unit with temporal_id equal to 0, unless the sequence parameterset is provided through external means. An activated sequence parameterset RBSP remains active for the entire coded video sequence.

Each video parameter set RBSP is initially considered not active at thestart of the operation of the decoding process. At most one videoparameter set RBSP is considered active at any given moment during theoperation of the decoding process, and the activation of any particularvideo parameter set RBSP results in the deactivation of thepreviously-active video parameter set RBSP (if any).

When a video parameter set RBSP (with a particular value ofvideo_parameter_set_id) is not already active and it is referred to byactivation of a sequence parameter set RBSP (using that value ofvideo_parameter_set_id), it is activated. This video parameter set RBSPis called the active video parameter set RBSP until it is deactivated bythe activation of another video parameter set RBSP. A video parameterset RBSP, with that particular value of video_parameter_set_id isavailable to the decoding process prior to its activation, included inat least one access unit with temporal_id equal to 0, unless the videoparameter set is provided through external means. An activated videoparameter set RBSP remains active for the entire coded video sequence.

During operation of the decoding process in a draft HEVC standard, thevalues of parameters of the active video parameter set, the activesequence parameter set, the active picture parameter set RBSP and theactive adaptation parameter set RBSP are considered in effect. Forinterpretation of SEI messages, the values of the active video parameterset, the active sequence parameter set, the active picture parameter setRBSP and the active adaptation parameter set RBSP for the operation ofthe decoding process for the VCL NAL units of the coded picture in thesame access unit are considered in effect unless otherwise specified inthe SEI message semantics.

Parameter set syntax structures of other types than those presentedearlier have also been proposed. In the following paragraphs, some ofthe proposed types of parameter sets are described.

It has been proposed that at least a subset of syntax elements that haveconventionally been included in a slice header are included in a GOS(Group of Slices) parameter set by an encoder. An encoder may code a GOSparameter set as a NAL unit. GOS parameter set NAL units may be includedin the bitstream together with for example coded slice NAL units, butmay also be carried out-of-band as described earlier in the context ofother parameter sets.

The GOS parameter set syntax structure may include an identifier, whichmay be used when referring to a particular GOS parameter set instancefor example from a slice header or another GOS parameter set.Alternatively, the GOS parameter set syntax structure does not includean identifier but an identifier may be inferred by both the encoder anddecoder for example using the bitstream order of GOS parameter setsyntax structures and a pre-defined numbering scheme.

The encoder and the decoder may infer the contents or the instance ofGOS parameter set from other syntax structures already encoded ordecoded or present in the bitstream. For example, the slice header ofthe texture view component of the base view may implicitly form a GOSparameter set. The encoder and decoder may infer an identifier value forsuch inferred GOS parameter sets. For example, the GOS parameter setformed from the slice header of the texture view component of the baseview may be inferred to have identifier value equal to 0.

A GOS parameter set may be valid within a particular access unitassociated with it. For example, if a GOS parameter set syntax structureis included in the NAL unit sequence for a particular access unit, wherethe sequence is in decoding or bitstream order, the GOS parameter setmay be valid from its appearance location until the end of the accessunit. Alternatively, a GOS parameter set may be valid for many accessunits.

The encoder may encode many GOS parameter sets for an access unit. Theencoder may determine to encode a GOS parameter set if it is known,expected, or estimated that at least a subset of syntax element valuesin a slice header to be coded would be the same in a subsequent sliceheader.

A limited numbering space may be used for the GOS parameter setidentifier. For example, a fixed-length code may be used and may beinterpreted as an unsigned integer value of a certain range. The encodermay use a GOS parameter set identifier value for a first GOS parameterset and subsequently for a second GOS parameter set, if the first GOSparameter set is subsequently not referred to for example by any sliceheader or GOS parameter set. The encoder may repeat a GOS parameter setsyntax structure within the bitstream for example to achieve a betterrobustness against transmission errors.

Syntax elements which may be included in a GOS parameter set may beconceptually collected in sets of syntax elements. A set of syntaxelements for a GOS parameter set may be formed for example on one ormore of the following basis:

-   -   Syntax elements indicating a scalable layer and/or other        scalability features    -   Syntax elements indicating a view and/or other multiview        features    -   Syntax elements related to a particular component type, such as        depth/disparity    -   Syntax elements related to access unit identification, decoding        order and/or output order and/or other syntax elements which may        stay unchanged for all slices of an access unit    -   Syntax elements which may stay unchanged in all slices of a view        component    -   Syntax elements related to reference picture list modification    -   Syntax elements related to the reference picture set used    -   Syntax elements related to decoding reference picture marking    -   Syntax elements related to prediction weight tables for weighted        prediction    -   Syntax elements for controlling deblocking filtering    -   Syntax elements for controlling adaptive loop filtering    -   Syntax elements for controlling sample adaptive offset    -   Any combination of sets above.

For each syntax element set, the encoder may have one or more of thefollowing options when coding a GOS parameter set:

-   -   The syntax element set may be coded into a GOS parameter set        syntax structure, i.e. coded syntax element values of the syntax        element set may be included in the GOS parameter set syntax        structure.    -   The syntax element set may be included by reference into a GOS        parameter set. The reference may be given as an identifier to        another GOS parameter set. The encoder may use a different        reference GOS parameter set for different syntax element sets.    -   The syntax element set may be indicated or inferred to be absent        from the GOS parameter set.

The options from which the encoder is able to choose for a particularsyntax element set when coding a GOS parameter set may depend on thetype of the syntax element set. For example, a syntax element setrelated to scalable layers may always be present in a GOS parameter set,while the set of syntax elements which may stay unchanged in all slicesof a view component may not be available for inclusion by reference butmay be optionally present in the GOS parameter set and the syntaxelements related to reference picture list modification may be includedby reference in, included as such in, or be absent from a GOS parameterset syntax structure. The encoder may encode indications in thebitstream, for example in a GOS parameter set syntax structure, whichoption was used in encoding. The code table and/or entropy coding maydepend on the type of the syntax element set. The decoder may use, basedon the type of the syntax element set being decoded, the code tableand/or entropy decoding that is matched with the code table and/orentropy encoding used by the encoder.

The encoder may have multiple means to indicate the association betweena syntax element set and the GOS parameter set used as the source forthe values of the syntax element set. For example, the encoder mayencode a loop of syntax elements where each loop entry is encoded assyntax elements indicating a GOS parameter set identifier value used asa reference and identifying the syntax element sets copied from thereference GOP parameter set. In another example, the encoder may encodea number of syntax elements, each indicating a GOS parameter set. Thelast GOS parameter set in the loop containing a particular syntaxelement set is the reference for that syntax element set in the GOSparameter set the encoder is currently encoding into the bitstream. Thedecoder parses the encoded GOS parameter sets from the bitstreamaccordingly so as to reproduce the same GOS parameter sets as theencoder.

A header parameter set (HPS) was proposed in document JCTVC-J0109(http://phenix.int-evey.fr/jct/doc_end_user/current_document.php?id=5972).An HPS is similar to GOS parameter set. A slice header is predicted fromone or more HPSs. In other words, the values of slice header syntaxelements can be selectively taken from one or more HPSs. If a pictureconsists of only one slice, the use of HPS is optional and a sliceheader can be included in the coded slice NAL unit instead. Twoalternative approaches of the HPS design were proposed in JCTVC-J109: asingle-AU HPS, where an HPS is applicable only to the slices within thesame assess unit, and a multi-AU HPS, where an HPS may be applicable toslices in multiple access units. The two proposed approaches are similarin their syntax. The main differences between the two approaches arisefrom the fact that the single-AU HPS design requires transmission of anHPS for each access unit, while the multi-AU HPS design allows re-use ofthe same HPS across multiple AUs.

A SEI NAL unit may contain one or more SEI messages, which are notrequired for the decoding of output pictures but may assist in relatedprocesses, such as picture output timing, rendering, error detection,error concealment, and resource reservation. Several SEI messages arespecified in H.264/AVC and HEVC, and the user data SEI messages enableorganizations and companies to specify SEI messages for their own use.H.264/AVC and HEVC contain the syntax and semantics for the specifiedSEI messages but no process for handling the messages in the recipientis defined. Consequently, encoders are required to follow the H.264/AVCstandard or the HEVC standard when they create SEI messages, anddecoders conforming to the H.264/AVC standard or the HEVC standard,respectively, are not required to process SEI messages for output orderconformance. One of the reasons to include the syntax and semantics ofSEI messages in H.264/AVC and HEVC is to allow different systemspecifications to interpret the supplemental information identically andhence interoperate. It is intended that system specifications canrequire the use of particular SEI messages both in the encoding end andin the decoding end, and additionally the process for handlingparticular SEI messages in the recipient can be specified.

A coded picture is a coded representation of a picture. A coded picturein H.264/AVC comprises the VCL NAL units that are required for thedecoding of the picture. In H.264/AVC, a coded picture can be a primarycoded picture or a redundant coded picture. A primary coded picture isused in the decoding process of valid bitstreams, whereas a redundantcoded picture is a redundant representation that should only be decodedwhen the primary coded picture cannot be successfully decoded. In adraft HEVC, no redundant coded picture has been specified.

In H.264/AVC, an access unit comprises a primary coded picture and thoseNAL units that are associated with it. In HEVC, an access unit isdefined as a set of NAL units that are associated with each otheraccording to a specified classification rule, are consecutive indecoding order, and contain exactly one coded picture. In H.264/AVC, theappearance order of NAL units within an access unit is constrained asfollows. An optional access unit delimiter NAL unit may indicate thestart of an access unit. It is followed by zero or more SEI NAL units.The coded slices of the primary coded picture appear next. In H.264/AVC,the coded slice of the primary coded picture may be followed by codedslices for zero or more redundant coded pictures. A redundant codedpicture is a coded representation of a picture or a part of a picture. Aredundant coded picture may be decoded if the primary coded picture isnot received by the decoder for example due to a loss in transmission ora corruption in physical storage medium.

In H.264/AVC, an access unit may also include an auxiliary codedpicture, which is a picture that supplements the primary coded pictureand may be used for example in the display process. An auxiliary codedpicture may for example be used as an alpha channel or alpha planespecifying the transparency level of the samples in the decodedpictures. An alpha channel or plane may be used in a layered compositionor rendering system, where the output picture is formed by overlayingpictures being at least partly transparent on top of each other. Anauxiliary coded picture has the same syntactic and semantic restrictionsas a monochrome redundant coded picture. In H.264/AVC, an auxiliarycoded picture contains the same number of macroblocks as the primarycoded picture.

In HEVC, an access unit may be defined as a set of NAL units that areassociated with each other according to a specified classification rule,are consecutive in decoding order, and contain exactly one codedpicture. In addition to containing the VCL NAL units of the codedpicture, an access unit may also contain non-VCL NAL units. In HEVC thedecoding of an access unit results in a decoded picture.

In H.264/AVC, a coded video sequence is defined to be a sequence ofconsecutive access units in decoding order from an IDR access unit,inclusive, to the next IDR access unit, exclusive, or to the end of thebitstream, whichever appears earlier. In a draft HEVC standard, a codedvideo sequence is defined to be a sequence of access units thatconsists, in decoding order, of a CRA access unit that is the firstaccess unit in the bitstream, an IDR access unit or a BLA access unit,followed by zero or more non-IDR and non-BLA access units including allsubsequent access units up to but not including any subsequent IDR orBLA access unit.

A group of pictures (GOP) and its characteristics may be defined asfollows. A GOP can be decoded regardless of whether any previouspictures were decoded. An open GOP is such a group of pictures in whichpictures preceding the initial intra picture in output order might notbe correctly decodable when the decoding starts from the initial intrapicture of the open GOP. In other words, pictures of an open GOP mayrefer (in inter prediction) to pictures belonging to a previous GOP. AnH.264/AVC decoder can recognize an intra picture starting an open GOPfrom the recovery point SEI message in an H.264/AVC bitstream. An HEVCdecoder can recognize an intra picture starting an open GOP, because aspecific NAL unit type, CRA NAL unit type, is used for its coded slices.A closed GOP is such a group of pictures in which all pictures can becorrectly decoded when the decoding starts from the initial intrapicture of the closed GOP. In other words, no picture in a closed GOPrefers to any pictures in previous GOPs. In H.264/AVC and HEVC, a closedGOP starts from an IDR access unit. In HEVC a closed GOP may also startfrom a BLA_W_DLP or a BLA_N_LP picture. As a result, closed GOPstructure has more error resilience potential in comparison to the openGOP structure, however at the cost of possible reduction in thecompression efficiency. Open GOP coding structure is potentially moreefficient in the compression, due to a larger flexibility in selectionof reference pictures.

A Structure of Pictures (SOP) may be defined as one or more codedpictures consecutive in decoding order, in which the first coded picturein decoding order is a reference picture at the lowest temporalsub-layer and no coded picture except potentially the first codedpicture in decoding order is a RAP picture. The relative decoding orderof the pictures is illustrated by the numerals inside the pictures. Anypicture in the previous SOP has a smaller decoding order than anypicture in the current SOP and any picture in the next SOP has a largerdecoding order than any picture in the current SOP. The term group ofpictures (GOP) may sometimes be used interchangeably with the term SOPand having the same semantics as the semantics of SOP rather than thesemantics of closed or open GOP as described above.

The bitstream syntax of H.264/AVC indicates whether a particular pictureis a reference picture, which may be used as a reference for interprediction of any other picture. In H.264/AVC, the NAL unit headerindicates the type of the NAL unit and whether a coded slice containedin the NAL unit is a part of a reference picture or a non-referencepicture. In HEVC, there are two NAL unit types for many picture types(e.g. TRAIL_R, TRAIL_N), differentiated whether the picture may be usedas reference for inter prediction in subsequent pictures in decodingorder in the same sub-layer. Sub-layer non-reference picture (oftendenoted by N in the picture type acronyms) may be defined as picturethat contains samples that cannot be used for inter prediction in thedecoding process of subsequent pictures of the same sub-layer indecoding order. Sub-layer non-reference pictures may be used asreference for pictures with a greater TemporalId value. Sub-layerreference picture (often denoted by _R in the picture type acronyms) maybe defined as picture that may be used as reference for inter predictionin the decoding process of subsequent pictures of the same sub-layer indecoding order. Pictures of any coding type (I, P, B) can be referencepictures or non-reference pictures in H.264/AVC and HEVC. Slices withina picture may have different coding types.

Many hybrid video codecs, including H.264/AVC and HEVC, encode videoinformation in two phases. In the first phase, predictive coding isapplied for example as so-called sample prediction and/or as so-calledsyntax prediction. In the sample prediction, pixel or sample values in acertain picture area or “block” are predicted. These pixel or samplevalues can be predicted, for example, using one or more of the followingways:

Motion compensation mechanisms (which may also be referred to astemporal prediction or motion-compensated temporal prediction ormotion-compensated prediction or MCP), which involve finding andindicating an area in one of the previously encoded video frames thatcorresponds closely to the block being coded.

Inter-view prediction, which involves finding and indicating an area inone of the previously encoded view components that corresponds closelyto the block being coded.

View synthesis prediction, which involves synthesizing a predictionblock or image area where a prediction block is derived on the basis ofreconstructed/decoded ranging information.

Inter-layer prediction using reconstructed/decoded samples, such as theso-called IntraBL (base layer) mode of SVC.

Inter-layer residual prediction, in which for example the coded residualof a reference layer or a derived residual from a difference of areconstructed/decoded reference layer picture and a correspondingreconstructed/decoded enhancement layer picture may be used forpredicting a residual block of the current enhancement layer block. Aresidual block may be added for example to a motion-compensatedprediction block to obtain a final prediction block for the currentenhancement layer block.

Intra prediction, where pixel or sample values can be predicted byspatial mechanisms which involve finding and indicating a spatial regionrelationship.

In the syntax prediction, which may also be referred to as parameterprediction, syntax elements and/or syntax element values and/orvariables derived from syntax elements are predicted from syntaxelements (de)coded earlier and/or variables derived earlier.Non-limiting examples of syntax prediction are provided below:

In motion vector prediction, motion vectors e.g. for inter and/orinter-view prediction may be coded differentially with respect to ablock-specific predicted motion vector. In many video codecs, thepredicted motion vectors are created in a predefined way, for example bycalculating the median of the encoded or decoded motion vectors of theadjacent blocks. Another way to create motion vector predictions,sometimes referred to as advanced motion vector prediction (AMVP), is togenerate a list of candidate predictions from adjacent blocks and/orco-located blocks in temporal reference pictures and signalling thechosen candidate as the motion vector predictor. In addition topredicting the motion vector values, the reference index of a previouslycoded/decoded picture can be predicted. The reference index may bepredicted from adjacent blocks and/or co-located blocks in temporalreference picture. Differential coding of motion vectors may be disabledacross slice boundaries.

The block partitioning, e.g. from CTU to CUs and down to PUs, may bepredicted.

In filter parameter prediction, the filtering parameters e.g. for sampleadaptive offset may be predicted.

Prediction approaches using image information from a previously codedimage can also be called as inter prediction methods which may also bereferred to as temporal prediction and motion compensation. Predictionapproaches using image information within the same image can also becalled as intra prediction methods.

The second phase is one of coding the error between the predicted blockof pixels or samples and the original block of pixels or samples. Thismay be accomplished by transforming the difference in pixel or samplevalues using a specified transform. This transform may be a DiscreteCosine Transform (DCT) or a variant thereof. After transforming thedifference, the transformed difference is quantized and entropy encoded.

By varying the fidelity of the quantization process, the encoder cancontrol the balance between the accuracy of the pixel or samplerepresentation (i.e. the visual quality of the picture) and the size ofthe resulting encoded video representation (i.e. the file size ortransmission bit rate).

The decoder reconstructs the output video by applying a predictionmechanism similar to that used by the encoder in order to form apredicted representation of the pixel or sample blocks (using the motionor spatial information created by the encoder and stored in thecompressed representation of the image) and prediction error decoding(the inverse operation of the prediction error coding to recover thequantized prediction error signal in the spatial domain).

After applying pixel or sample prediction and error decoding processesthe decoder may combine the prediction and the prediction error signals(the pixel or sample values) to form the output video frame.

The decoder (and encoder) may also apply additional filtering processesin order to improve the quality of the output video before passing itfor display and/or storing as a prediction reference for the forthcomingpictures in the video sequence.

In many video codecs, including H.264/AVC and HEVC, motion informationis indicated by motion vectors associated with each motion compensatedimage block. Each of these motion vectors represents the displacement ofthe image block in the picture to be coded (in the encoder) or decoded(at the decoder) and the prediction source block in one of thepreviously coded or decoded images (or pictures). H.264/AVC and HEVC, asmany other video compression standards, divide a picture into a mesh ofrectangles, for each of which a similar block in one of the referencepictures is indicated for inter prediction. The location of theprediction block is coded as a motion vector that indicates the positionof the prediction block relative to the block being coded.

Inter prediction process may be characterized for example using one ormore of the following factors.

The Accuracy of Motion Vector Representation.

For example, motion vectors may be of quarter-pixel accuracy, half-pixelaccuracy or full-pixel accuracy and sample values in fractional-pixelpositions may be obtained using a finite impulse response (FIR) filter.

Block Partitioning for Inter Prediction.

Many coding standards, including H.264/AVC and HEVC, allow selection ofthe size and shape of the block for which a motion vector is applied formotion-compensated prediction in the encoder, and indicating theselected size and shape in the bitstream so that decoders can reproducethe motion-compensated prediction done in the encoder. This block mayalso be referred to as a motion partition.

Number of Reference Pictures for Inter Prediction.

The sources of inter prediction are previously decoded pictures. Manycoding standards, including H.264/AVC and HEVC, enable storage ofmultiple reference pictures for inter prediction and selection of theused reference picture on a block basis. For example, reference picturesmay be selected on macroblock or macroblock partition basis in H.264/AVCand on PU or CU basis in HEVC. Many coding standards, such as H.264/AVCand HEVC, include syntax structures in the bitstream that enabledecoders to create one or more reference picture lists. A referencepicture index to a reference picture list may be used to indicate whichone of the multiple reference pictures is used for inter prediction fora particular block. A reference picture index may be coded by an encoderinto the bitstream in some inter coding modes or it may be derived (byan encoder and a decoder) for example using neighboring blocks in someother inter coding modes.

Motion Vector Prediction.

In order to represent motion vectors efficiently in bitstreams, motionvectors may be coded differentially with respect to a block-specificpredicted motion vector. In many video codecs, the predicted motionvectors are created in a predefined way, for example by calculating themedian of the encoded or decoded motion vectors of the adjacent blocks.Another way to create motion vector predictions, sometimes referred toas advanced motion vector prediction (AMVP), is to generate a list ofcandidate predictions from adjacent blocks and/or co-located blocks intemporal reference pictures and signalling the chosen candidate as themotion vector predictor. In addition to predicting the motion vectorvalues, the reference index of previously coded/decoded picture can bepredicted. The reference index may be predicted from adjacent blocksand/or co-located blocks in temporal reference picture. Differentialcoding of motion vectors may be disabled across slice boundaries.

Multi-Hypothesis Motion-Compensated Prediction.

H.264/AVC and HEVC enable the use of a single prediction block in Pslices (herein referred to as uni-predictive slices) or a linearcombination of two motion-compensated prediction blocks forbi-predictive slices, which are also referred to as B slices. Individualblocks in B slices may be bi-predicted, uni-predicted, orintra-predicted, and individual blocks in P slices may be uni-predictedor intra-predicted. The reference pictures for a bi-predictive picturemay not be limited to be the subsequent picture and the previous picturein output order, but rather any reference pictures may be used. In manycoding standards, such as H.264/AVC and HEVC, one reference picturelist, referred to as reference picture list 0, is constructed for Pslices, and two reference picture lists, list 0 and list 1, areconstructed for B slices. For B slices, when prediction in forwarddirection may refer to prediction from a reference picture in referencepicture list 0, and prediction in backward direction may refer toprediction from a reference picture in reference picture list 1, eventhough the reference pictures for prediction may have any decoding oroutput order relation to each other or to the current picture.

Weighted Prediction.

Many coding standards use a prediction weight of 1 for prediction blocksof inter (P) pictures and 0.5 for each prediction block of a B picture(resulting into averaging). H.264/AVC allows weighted prediction forboth P and B slices. In implicit weighted prediction, the weights areproportional to picture order counts, while in explicit weightedprediction, prediction weights are explicitly indicated. The weights forexplicit weighted prediction may be indicated for example in one or moreof the following syntax structure: a slice header, a picture header, apicture parameter set, an adaptation parameter set or any similar syntaxstructure.

In many video codecs, the prediction residual after motion compensationis first transformed with a transform kernel (like DCT) and then coded.The reason for this is that often there still exists some correlationamong the residual and transform can in many cases help reduce thiscorrelation and provide more efficient coding.

In some coding formats and codecs, a distinction is made betweenso-called short-term and long-term reference pictures.

Some video coding formats, such as H.264/AVC, include the frame_numsyntax element, which is used for various decoding processes related tomultiple reference pictures. In H.264/AVC, the value of frame_num forIDR pictures is 0. The value of frame_num for non-IDR pictures is equalto the frame_num of the previous reference picture in decoding orderincremented by 1 (in modulo arithmetic, i.e., the value of frame_numwrap over to 0 after a maximum value of frame_num).

H.264/AVC and HEVC include a concept of picture order count (POC). Avalue of POC is derived for each picture and is non-decreasing withincreasing picture position in output order. POC therefore indicates theoutput order of pictures. POC may be used in the decoding process forexample for implicit scaling of motion vectors in the temporal directmode of bi-predictive slices, for implicitly derived weights in weightedprediction, and for reference picture list initialization. Furthermore,POC may be used in the verification of output order conformance. InH.264/AVC, POC is specified relative to the previous IDR picture or apicture containing a memory management control operation marking allpictures as “unused for reference”.

A syntax structure for decoded reference picture marking may exist in avideo coding system. For example, when the decoding of the picture hasbeen completed, the decoded reference picture marking syntax structure,if present, may be used to adaptively mark pictures as “unused forreference” or “used for long-term reference”. If the decoded referencepicture marking syntax structure is not present and the number ofpictures marked as “used for reference” can no longer increase, asliding window reference picture marking may be used, which basicallymarks the earliest (in decoding order) decoded reference picture asunused for reference.

H.264/AVC specifies the process for decoded reference picture marking inorder to control the memory consumption in the decoder. The maximumnumber of reference pictures used for inter prediction, referred to asM, is determined in the sequence parameter set. When a reference pictureis decoded, it is marked as “used for reference”. If the decoding of thereference picture caused more than M pictures marked as “used forreference”, at least one picture is marked as “unused for reference”.There are two types of operation for decoded reference picture marking:adaptive memory control and sliding window. The operation mode fordecoded reference picture marking is selected on picture basis. Theadaptive memory control enables explicit signaling which pictures aremarked as “unused for reference” and may also assign long-term indicesto short-term reference pictures. The adaptive memory control mayrequire the presence of memory management control operation (MMCO)parameters in the bitstream. MMCO parameters may be included in adecoded reference picture marking syntax structure. If the slidingwindow operation mode is in use and there are M pictures marked as “usedfor reference”, the short-term reference picture that was the firstdecoded picture among those short-term reference pictures that aremarked as “used for reference” is marked as “unused for reference”. Inother words, the sliding window operation mode results intofirst-in-first-out buffering operation among short-term referencepictures.

One of the memory management control operations in H.264/AVC causes allreference pictures except for the current picture to be marked as“unused for reference”. An instantaneous decoding refresh (IDR) picturecontains only intra-coded slices and causes a similar “reset” ofreference pictures.

In a draft HEVC standard, reference picture marking syntax structuresand related decoding processes are not used, but instead a referencepicture set (RPS) syntax structure and decoding process are used insteadfor a similar purpose. A reference picture set valid or active for apicture includes all the reference pictures used as a reference for thepicture and all the reference pictures that are kept marked as “used forreference” for any subsequent pictures in decoding order. There are sixsubsets of the reference picture set, which are referred to as namelyRefPicSetStCurr0 (which may also or alternatively referred to asRefPicSetStCurrBefore), RefPicSetStCurr1 (which may also oralternatively referred to as RefPicSetStCurrAfter), RefPicSetStFoll0,RefPicSetStFoll1, RefPicSetLtCurr, and RefPicSetLtFoll. In some HEVCdraft specifications, RefPicSetStFoll0 and RefPicSetStFoll1 are regardedas one subset, which may be referred to as RefPicSetStFoll. The notationof the six subsets is as follows. “Curr” refers to reference picturesthat are included in the reference picture lists of the current pictureand hence may be used as inter prediction reference for the currentpicture. “Foll” refers to reference pictures that are not included inthe reference picture lists of the current picture but may be used insubsequent pictures in decoding order as reference pictures. “St” refersto short-term reference pictures, which may generally be identifiedthrough a certain number of least significant bits of their POC value.“Lt” refers to long-term reference pictures, which are specificallyidentified and generally have a greater difference of POC valuesrelative to the current picture than what can be represented by thementioned certain number of least significant bits. “0” refers to thosereference pictures that have a smaller POC value than that of thecurrent picture. “1” refers to those reference pictures that have agreater POC value than that of the current picture. RefPicSetStCurr0,RefPicSetStCurr1, RefPicSetStFoll0 and RefPicSetStFoll1 are collectivelyreferred to as the short-term subset of the reference picture set.RefPicSetLtCurr and RefPicSetLtFoll are collectively referred to as thelong-term subset of the reference picture set.

In a draft HEVC standard, a reference picture set may be specified in asequence parameter set and taken into use in the slice header through anindex to the reference picture set. A reference picture set may also bespecified in a slice header. A long-term subset of a reference pictureset is generally specified only in a slice header, while the short-termsubsets of the same reference picture set may be specified in thepicture parameter set or slice header. A reference picture set may becoded independently or may be predicted from another reference pictureset (known as inter-RPS prediction). When a reference picture set isindependently coded, the syntax structure includes up to three loopsiterating over different types of reference pictures; short-termreference pictures with lower POC value than the current picture,short-term reference pictures with higher POC value than the currentpicture and long-term reference pictures. Each loop entry specifies apicture to be marked as “used for reference”. In general, the picture isspecified with a differential POC value. The inter-RPS predictionexploits the fact that the reference picture set of the current picturecan be predicted from the reference picture set of a previously decodedpicture. This is because all the reference pictures of the currentpicture are either reference pictures of the previous picture or thepreviously decoded picture itself. It is only necessary to indicatewhich of these pictures should be reference pictures and be used for theprediction of the current picture. In both types of reference pictureset coding, a flag (used by curr_pic_X_flag) is additionally sent foreach reference picture indicating whether the reference picture is usedfor reference by the current picture (included in a *Curr list) or not(included in a *Foll list). Pictures that are included in the referencepicture set used by the current slice are marked as “used forreference”, and pictures that are not in the reference picture set usedby the current slice are marked as “unused for reference”. If thecurrent picture is an IDR picture, RefPicSetStCurr0, RefPicSetStCurr1,RefPicSetStFoll0, RefPicSetStFoll1, RefPicSetLtCurr, and RefPicSetLtFollare all set to empty.

A Decoded Picture Buffer (DPB) may be used in the encoder and/or in thedecoder. There are two reasons to buffer decoded pictures, forreferences in inter prediction and for reordering decoded pictures intooutput order. As H.264/AVC and HEVC provide a great deal of flexibilityfor both reference picture marking and output reordering, separatebuffers for reference picture buffering and output picture buffering maywaste memory resources. Hence, the DPB may include a unified decodedpicture buffering process for reference pictures and output reordering.A decoded picture may be removed from the DPB when it is no longer usedas a reference and is not needed for output.

In many coding modes of H.264/AVC and HEVC, the reference picture forinter prediction is indicated with an index to a reference picture list.The index may be coded with variable length coding, which usually causesa smaller index to have a shorter value for the corresponding syntaxelement. In H.264/AVC and HEVC, two reference picture lists (referencepicture list 0 and reference picture list 1) are generated for eachbi-predictive (B) slice, and one reference picture list (referencepicture list 0) is formed for each inter-coded (P) slice. In addition,for a B slice in a draft HEVC standard, a combined list (List C) isconstructed after the final reference picture lists (List 0 and List 1)have been constructed. The combined list may be used for uni-prediction(also known as uni-directional prediction) within B slices. However, inthe final H.265/HEVC standard, no combined list is constructed.

A reference picture list, such as reference picture list 0 and referencepicture list 1, may be constructed in two steps: First, an initialreference picture list is generated. The initial reference picture listmay be generated for example on the basis of frame_num, POC,temporal_id, or information on the prediction hierarchy such as GOPstructure, or any combination thereof. Second, the initial referencepicture list may be reordered by reference picture list reordering(RPLR) commands, also known as reference picture list modificationsyntax structure, which may be contained in slice headers. The RPLRcommands indicate the pictures that are ordered to the beginning of therespective reference picture list. This second step may also be referredto as the reference picture list modification process, and the RPLRcommands may be included in a reference picture list modification syntaxstructure. If reference picture sets are used, the reference picturelist 0 may be initialized to contain RefPicSetStCurr0 first, followed byRefPicSetStCurr1, followed by RefPicSetLtCurr. Reference picture list 1may be initialized to contain RefPicSetStCurr1 first, followed byRefPicSetStCurr0. The initial reference picture lists may be modifiedthrough the reference picture list modification syntax structure, wherepictures in the initial reference picture lists may be identifiedthrough an entry index to the list.

The combined list in a draft HEVC standard may be constructed asfollows. If the modification flag for the combined list is zero, thecombined list is constructed by an implicit mechanism; otherwise it isconstructed by reference picture combination commands included in thebitstream. In the implicit mechanism, reference pictures in List C aremapped to reference pictures from List 0 and List 1 in an interleavedfashion starting from the first entry of List 0, followed by the firstentry of List 1 and so forth. Any reference picture that has alreadybeen mapped in List C is not mapped again. In the explicit mechanism,the number of entries in List C is signaled, followed by the mappingfrom an entry in List 0 or List 1 to each entry of List C. In addition,when List 0 and List 1 are identical the encoder has the option ofsetting the ref_pic_list_combination_flag to 0 to indicate that noreference pictures from List 1 are mapped, and that List C is equivalentto List 0.

The advanced motion vector prediction (AMVP) may operate for example asfollows, while other similar realizations of advanced motion vectorprediction are also possible for example with different candidateposition sets and candidate locations with candidate position sets. Twospatial motion vector predictors (MVPs) may be derived and a temporalmotion vector predictor (TMVP) may be derived. They may be selectedamong the positions shown in FIG. 8: three spatial motion vectorpredictor candidate positions 103, 104, 105 located above the currentprediction block 100 (B0, B1, B2) and two 101, 102 on the left (A0, A1).The first motion vector predictor that is available (e.g. resides in thesame slice, is inter-coded, etc.) in a pre-defined order of eachcandidate position set, (B0, B1, B2) or (A0, A1), may be selected torepresent that prediction direction (up or left) in the motion vectorcompetition. A reference index for the temporal motion vector predictormay be indicated by the encoder in the slice header (e.g. as acollocated_ref_idx syntax element). The motion vector obtained from theco-located picture may be scaled according to the proportions of thepicture order count differences of the reference picture of the temporalmotion vector predictor, the co-located picture, and the currentpicture. Moreover, a redundancy check may be performed among thecandidates to remove identical candidates, which can lead to theinclusion of a zero motion vector in the candidate list. The motionvector predictor may be indicated in the bitstream for example byindicating the direction of the spatial motion vector predictor (up orleft) or the selection of the temporal motion vector predictorcandidate.

In addition to predicting the motion vector values, the reference indexof previously coded/decoded picture can be predicted. The referenceindex may be predicted from adjacent blocks and/or from co-locatedblocks in a temporal reference picture.

Many high efficiency video codecs such as a draft HEVC codec employ anadditional motion information coding/decoding mechanism, often calledmerging/merge mode/process/mechanism, where all the motion informationof a block/PU is predicted and used without any modification/correction.The aforementioned motion information for a PU may comprise one or moreof the following: 1) The information whether ‘the PU is uni-predictedusing only reference picture list0’ or ‘the PU is uni-predicted usingonly reference picture list1’ or ‘the PU is bi-predicted using bothreference picture list0 and list1’; 2) Motion vector value correspondingto the reference picture list0, which may comprise a horizontal andvertical motion vector component; 3) Reference picture index in thereference picture list0 and/or an identifier of a reference picturepointed to by the Motion vector corresponding to reference picture list0, where the identifier of a reference picture may be for example apicture order count value, a layer identifier value (for inter-layerprediction), or a pair of a picture order count value and a layeridentifier value; 4) Information of the reference picture marking of thereference picture, e.g. information whether the reference picture wasmarked as “used for short-term reference” or “used for long-termreference”; 5)-7) The same as 2)-4), respectively, but for referencepicture list1.

Similarly, predicting the motion information is carried out using themotion information of adjacent blocks and/or co-located blocks intemporal reference pictures. A list, often called as a merge list, maybe constructed by including motion prediction candidates associated withavailable adjacent/co-located blocks and the index of selected motionprediction candidate in the list is signalled and the motion informationof the selected candidate is copied to the motion information of thecurrent PU. When the merge mechanism is employed for a whole CU and theprediction signal for the CU is used as the reconstruction signal, i.e.prediction residual is not processed, this type of coding/decoding theCU is typically named as skip mode or merge based skip mode. In additionto the skip mode, the merge mechanism may also be employed forindividual PUs (not necessarily the whole CU as in skip mode) and inthis case, prediction residual may be utilized to improve predictionquality. This type of prediction mode is typically named as aninter-merge mode.

One of the candidates in the merge list may be a TMVP candidate, whichmay be derived from the collocated block within an indicated or inferredreference picture, such as the reference picture indicated for examplein the slice header for example using the collocated_ref_idx syntaxelement or alike.

In HEVC the so-called target reference index for temporal motion vectorprediction in the merge list is set as 0 when the motion coding mode isthe merge mode. When the motion coding mode in HEVC utilizing thetemporal motion vector prediction is the advanced motion vectorprediction mode, the target reference index values are explicitlyindicated (e.g. per each PU).

When the target reference index value has been determined, the motionvector value of the temporal motion vector prediction may be derived asfollows: Motion vector at the block that is co-located with thebottom-right neighbor of the current prediction unit is calculated. Thepicture where the co-located block resides may be e.g. determinedaccording to the signalled reference index in the slice header asdescribed above. The determined motion vector at the co-located block isscaled with respect to the ratio of a first picture order countdifference and a second picture order count difference. The firstpicture order count difference is derived between the picture containingthe co-located block and the reference picture of the motion vector ofthe co-located block. The second picture order count difference isderived between the current picture and the target reference picture. Ifone but not both of the target reference picture and the referencepicture of the motion vector of the co-located block is a long-termreference picture (while the other is a short-term reference picture),the TMVP candidate may be considered unavailable. If both of the targetreference picture and the reference picture of the motion vector of theco-located block are long-term reference pictures, no POC-based motionvector scaling may be applied.

Motion parameter types or motion information may include but are notlimited to one or more of the following types:

-   -   an indication of a prediction type (e.g. intra prediction,        uni-prediction, bi-prediction) and/or a number of reference        pictures;    -   an indication of a prediction direction, such as inter (a.k.a.        temporal) prediction, inter-layer prediction, inter-view        prediction, view synthesis prediction (VSP), and inter-component        prediction (which may be indicated per reference picture and/or        per prediction type and where in some embodiments inter-view and        view-synthesis prediction may be jointly considered as one        prediction direction) and/or    -   an indication of a reference picture type, such as a short-term        reference picture and/or a long-term reference picture and/or an        inter-layer reference picture (which may be indicated e.g. per        reference picture)    -   a reference index to a reference picture list and/or any other        identifier of a reference picture (which may be indicated e.g.        per reference picture and the type of which may depend on the        prediction direction and/or the reference picture type and which        may be accompanied by other relevant pieces of information, such        as the reference picture list or alike to which reference index        applies);    -   a horizontal motion vector component (which may be indicated        e.g. per prediction block or per reference index or alike);    -   a vertical motion vector component (which may be indicated e.g.        per prediction block or per reference index or alike);    -   one or more parameters, such as picture order count difference        and/or a relative camera separation between the picture        containing or associated with the motion parameters and its        reference picture, which may be used for scaling of the        horizontal motion vector component and/or the vertical motion        vector component in one or more motion vector prediction        processes (where said one or more parameters may be indicated        e.g. per each reference picture or each reference index or        alike);    -   coordinates of a block to which the motion parameters and/or        motion information applies, e.g. coordinates of the top-left        sample of the block in luma sample units; extents (e.g. a width        and a height) of a block to which the motion parameters and/or        motion information applies.

A motion field associated with a picture may be considered to compriseof a set of motion information produced for every coded block of thepicture. A motion field may be accessible by coordinates of a block, forexample. A motion field may be used for example in TMVP or any othermotion prediction mechanism where a source or a reference for predictionother than the current (de)coded picture is used.

Different spatial granularity or units may be applied to representand/or store a motion field. For example, a regular grid of spatialunits may be used. For example, a picture may be divided intorectangular blocks of certain size (with the possible exception ofblocks at the edges of the picture, such as on the right edge and thebottom edge). For example, the size of the spatial unit may be equal tothe smallest size for which a distinct motion can be indicated by theencoder in the bitstream, such as a 4×4 block in luma sample units. Forexample, a so-called compressed motion field may be used, where thespatial unit may be equal to a pre-defined or indicated size, such as a16×16 block in luma sample units, which size may be greater than thesmallest size for indicating distinct motion. For example, an HEVCencoder and/or decoder may be implemented in a manner that a motion datastorage reduction (MDSR) is performed for each decoded motion field(prior to using the motion field for any prediction between pictures).In an HEVC implementation, MDSR may reduce the granularity of motiondata to 16×16 blocks in luma sample units by keeping the motionapplicable to the top-left sample of the 16×16 block in the compressedmotion field. The encoder may encode indication(s) related to thespatial unit of the compressed motion field as one or more syntaxelements and/or syntax element values for example in a sequence-levelsyntax structure, such as a video parameter set or a sequence parameterset. In some (de)coding methods and/or devices, a motion field may berepresented and/or stored according to the block partitioning of themotion prediction (e.g. according to prediction units of the HEVCstandard). In some (de)coding methods and/or devices, a combination of aregular grid and block partitioning may be applied so that motionassociated with partitions greater than a pre-defined or indicatedspatial unit size is represented and/or stored associated with thosepartitions, whereas motion associated with partitions smaller than orunaligned with a pre-defined or indicated spatial unit size or grid isrepresented and/or stored for the pre-defined or indicated units.

Inter-Picture Motion Vector Prediction and its Relation to ScalableVideo Coding

Multi-view coding has been realized as a multi-loop scalable videocoding scheme, where the inter-view reference pictures are added intothe reference picture lists. In MVC the inter-view reference componentsand inter-view only reference components that are included in thereference picture lists may be considered as not being marked as “usedfor short-term reference” or “used for long-term reference”. In thederivation of temporal direct luma motion vector, the co-located motionvector may not be scaled if the picture order count difference of List 1reference (from which the co-located motion vector is obtained) and List0 reference is 0, i.e. if td is equal to 0 in FIG. 6 c.

FIG. 6a illustrates an example of spatial and temporal prediction of aprediction unit. There is depicted the current block 601 in the frame600 and a neighbor block 602 which already has been encoded. A motionvector definer 362 (FIG. 4) has defined a motion vector 603 for theneighbor block 602 which points to a block 604 in the previous frame605. This motion vector can be used as a potential spatial motion vectorprediction 610 for the current block. FIG. 6a depicts that a co-locatedblock 606 in the previous frame 605, i.e. the block at the same locationthan the current block but in the previous frame, has a motion vector607 pointing to a block 609 in another frame 608. This motion vector 607can be used as a potential temporal motion vector prediction 611 for thecurrent block 601.

FIG. 6b illustrates another example of spatial and temporal predictionof a prediction unit. In this example the block 606 of the previousframe 605 uses bi-directional prediction based on the block 609 of theframe 608 preceding the frame 605 and on the block 612 in the frame 613succeeding the current frame 600. The temporal motion vector predictionfor the current block 601 may be formed by using both motion vectors607, 614 or either of them.

In HEVC temporal motion vector prediction (TMVP), the reference picturelist to be used for obtaining a collocated partition is chosen accordingto the collocated_from_l0_flag syntax element in the slice header. Whenthe flag is equal to 1, it specifies that the picture that contains thecollocated partition is derived from list 0, otherwise the picture isderived from list 1. When collocated_from_l0_flag is not present, it isinferred to be equal to 1. The collocated_ref_idx in the slice headerspecifies the reference index of the picture that contains thecollocated partition. When the current slice is a P slice,collocated_ref_idx refers to a picture in list 0. When the current sliceis a B slice, collocated_ref_idx refers to a picture in list 0 ifcollocated_from_l0 is 1, otherwise it refers to a picture in list 1.collocated_ref_idx always refers to a valid list entry, and theresulting picture is the same for all slices of a coded picture. Whencollocated_ref_idx is not present, it is inferred to be equal to 0.

In HEVC, when the current PU uses the merge mode, the target referenceindex for TMVP is set to 0 (for both reference picture list 0 and 1). InAMVP, the target reference index is indicated in the bitstream.

In HEVC, the availability of a candidate predicted motion vector (PMV)for the merge mode may be determined as follows (both for spatial andtemporal candidates) (SRTP=short-term reference picture, LRTP=long-termreference picture)

reference picture for target reference picture for candidate PMVreference index candidate PMV availability STRP STRP “available” (andscaled) STRP LTRP “unavailable” LTRP STRP “unavailable” LTRP LTRP“available” but not scaled

Motion vector scaling may be performed in the case both target referencepicture and the reference index for candidate PMV are short-termreference pictures. The scaling may be performed by scaling the motionvector with appropriate POC differences related to the candidate motionvector and the target reference picture relative to the current picture,e.g. with the POC difference of the current picture and the targetreference picture divided by the POC difference of the current pictureand the POC difference of the picture containing the candidate PMV andits reference picture.

In FIG. 9a illustrating the operation of the HEVC merge mode formultiview video (e.g. MV-HEVC), the motion vector in the co-located PU,if referring to a short-term (ST) reference picture, is scaled to form amerge candidate of the current PU (PU0), wherein MV0 is scaled to MV0′during the merge mode. However, if the co-located PU has a motion vector(MV1) referring to an inter-view reference picture, marked as long-term,the motion vector is not used to predict the current PU (PU1), as thereference picture corresponding to reference index 0 is a short termreference picture and the reference picture of the candidate PMV is along-term reference picture.

It has been proposed that a new additional reference index (ref_idxAdd., also referred to as refIdxAdditional) may be derived so that themotion vectors referring to a long-term reference picture can be used toform a merge candidate and not considered as unavailable (when ref_idx 0points to a short-term picture). If ref_idx 0 points to a short-termreference picture, refIdxAdditional is set to point to the firstlong-term picture in the reference picture list. Vice versa, if ref_idx0 points to a long-term picture, refIdxAdditional is set to point to thefirst short-term reference picture in the reference picture list.refIdxAdditional is used in the merge mode instead of ref_idx 0 if its“type” (long-term or short-term) matches to that of the co-locatedreference index. An example of this is illustrated in FIG. 9 b.

Scalable video coding refers to a coding structure where one bitstreamcan contain multiple representations of the content at differentbitrates, resolutions and/or frame rates. In these cases the receivercan extract the desired representation depending on its characteristics(e.g. resolution that matches best with the resolution of the display ofthe device). Alternatively, a server or a network element can extractthe portions of the bitstream to be transmitted to the receiverdepending on e.g. the network characteristics or processing capabilitiesof the receiver.

A scalable bitstream may consist of a base layer providing the lowestquality video available and one or more enhancement layers that enhancethe video quality when received and decoded together with the lowerlayers. An enhancement layer may enhance the temporal resolution (i.e.,the frame rate), the spatial resolution, or simply the quality of thevideo content represented by another layer or part thereof. In order toimprove coding efficiency for the enhancement layers, the codedrepresentation of that layer may depend on the lower layers. Forexample, the motion and mode information of the enhancement layer can bepredicted from lower layers. Similarly the pixel data of the lowerlayers can be used to create prediction for the enhancement layer(s).

Each scalable layer together with all its dependent layers is onerepresentation of the video signal at a certain spatial resolution,temporal resolution and quality level. In this document, we refer to ascalable layer together with all of its dependent layers as a “scalablelayer representation”. The portion of a scalable bitstream correspondingto a scalable layer representation can be extracted and decoded toproduce a representation of the original signal at certain fidelity.

In some cases, data in an enhancement layer can be truncated after acertain location, or even at arbitrary positions, where each truncationposition may include additional data representing increasingly enhancedvisual quality. Such scalability is referred to as fine-grained(granularity) scalability (FGS). FGS was included in some draft versionsof the SVC standard, but it was eventually excluded from the final SVCstandard. FGS is subsequently discussed in the context of some draftversions of the SVC standard. The scalability provided by thoseenhancement layers that cannot be truncated is referred to ascoarse-grained (granularity) scalability (CGS). It collectively includesthe traditional quality (SNR) scalability and spatial scalability. TheSVC standard supports the so-called medium-grained scalability (MGS),where quality enhancement pictures are coded similarly to SNR scalablelayer pictures but indicated by high-level syntax elements similarly toFGS layer pictures, by having the quality_id syntax element greater than0.

SVC uses an inter-layer prediction mechanism, wherein certaininformation can be predicted from layers other than the currentlyreconstructed layer or the next lower layer. Information that could beinter-layer predicted includes intra texture, motion and residual data.Inter-layer motion prediction includes the prediction of block codingmode, header information, etc., wherein motion from the lower layer maybe used for prediction of the higher layer. In case of intra coding, aprediction from surrounding macroblocks or from co-located macroblocksof lower layers is possible. These prediction techniques do not employinformation from earlier coded access units and hence, are referred toas intra prediction techniques. Furthermore, residual data from lowerlayers can also be employed for prediction of the current layer, whichmay be referred to as inter-layer residual prediction.

Scalable video (de)coding may be realized with a concept known assingle-loop decoding, where decoded reference pictures are reconstructedonly for the highest layer being decoded while pictures at lower layersmay not be fully decoded or may be discarded after using them forinter-layer prediction. In single-loop decoding, the decoder performsmotion compensation and full picture reconstruction only for thescalable layer desired for playback (called the “desired layer” or the“target layer”), thereby reducing decoding complexity when compared tomulti-loop decoding. All of the layers other than the desired layer donot need to be fully decoded because all or part of the coded picturedata is not needed for reconstruction of the desired layer. However,lower layers (than the target layer) may be used for inter-layer syntaxor parameter prediction, such as inter-layer motion prediction.Additionally or alternatively, lower layers may be used for inter-layerintra prediction and hence intra-coded blocks of lower layers may haveto be decoded. Additionally or alternatively, inter-layer residualprediction may be applied, where the residual information of the lowerlayers may be used for decoding of the target layer and the residualinformation may need to be decoded or reconstructed. In some codingarrangements, a single decoding loop is needed for decoding of mostpictures, while a second decoding loop may be selectively applied toreconstruct so-called base representations (i.e. decoded base layerpictures), which may be needed as prediction references but not foroutput or display.

SVC allows the use of single-loop decoding. It is enabled by using aconstrained intra texture prediction mode, whereby the inter-layer intratexture prediction can be applied to macroblocks (MBs) for which thecorresponding block of the base layer is located inside intra-MBs. Atthe same time, those intra-MBs in the base layer use constrainedintra-prediction (e.g., having the syntax element“constrained_intrapred_flag” equal to 1). In single-loop decoding, thedecoder performs motion compensation and full picture reconstructiononly for the scalable layer desired for playback (called the “desiredlayer” or the “target layer”), thereby greatly reducing decodingcomplexity. All of the layers other than the desired layer do not needto be fully decoded because all or part of the data of the MBs not usedfor inter-layer prediction (be it inter-layer intra texture prediction,inter-layer motion prediction or inter-layer residual prediction) is notneeded for reconstruction of the desired layer. A single decoding loopis needed for decoding of most pictures, while a second decoding loop isselectively applied to reconstruct the base representations, which areneeded as prediction references but not for output or display, and arereconstructed only for the so called key pictures (for which“store_ref_base_pic_flag” is equal to 1).

The scalability structure in the SVC draft is characterized by threesyntax elements: “temporal_id,” “dependency_id” and “quality_id.” Thesyntax element “temporal_id” is used to indicate the temporalscalability hierarchy or, indirectly, the frame rate. A scalable layerrepresentation comprising pictures of a smaller maximum “temporal_id”value has a smaller frame rate than a scalable layer representationcomprising pictures of a greater maximum “temporal_id”. A given temporallayer typically depends on the lower temporal layers (i.e., the temporallayers with smaller “temporal_id” values) but does not depend on anyhigher temporal layer. The syntax element “dependency_id” is used toindicate the CGS inter-layer coding dependency hierarchy (which, asmentioned earlier, includes both SNR and spatial scalability). At anytemporal level location, a picture of a smaller “dependency_id” valuemay be used for inter-layer prediction for coding of a picture with agreater “dependency_id” value. The syntax element “quality_id” is usedto indicate the quality level hierarchy of a FGS or MGS layer. At anytemporal location, and with an identical “dependency_id” value, apicture with “quality_id” equal to QL uses the picture with “quality_id”equal to QL−1 for inter-layer prediction. A coded slice with“quality_id” larger than 0 may be coded as either a truncatable FGSslice or a non-truncatable MGS slice.

For simplicity, all the data units (e.g., Network Abstraction Layerunits or NAL units in the SVC context) in one access unit havingidentical value of “dependency_id” are referred to as a dependency unitor a dependency representation. Within one dependency unit, all the dataunits having identical value of “quality_id” are referred to as aquality unit or layer representation.

A base representation, also known as a decoded base picture, is adecoded picture resulting from decoding the Video Coding Layer (VCL) NALunits of a dependency unit having “quality_id” equal to 0 and for whichthe “store_ref_base_pic_flag” is set equal to 1. An enhancementrepresentation, also referred to as a decoded picture, results from theregular decoding process in which all the layer representations that arepresent for the highest dependency representation are decoded.

As mentioned earlier, CGS includes both spatial scalability and SNRscalability. Spatial scalability is initially designed to supportrepresentations of video with different resolutions. For each timeinstance, VCL NAL units are coded in the same access unit and these VCLNAL units can correspond to different resolutions. During the decoding,a low resolution VCL NAL unit provides the motion field and residualwhich can be optionally inherited by the final decoding andreconstruction of the high resolution picture. When compared to oldervideo compression standards, SVC's spatial scalability has beengeneralized to enable the base layer to be a cropped and zoomed versionof the enhancement layer.

MGS quality layers are indicated with “quality_id” similarly as FGSquality layers. For each dependency unit (with the same“dependency_id”), there is a layer with “quality_id” equal to 0 andthere can be other layers with “quality_id” greater than 0. These layerswith “quality_id” greater than 0 are either MGS layers or FGS layers,depending on whether the slices are coded as truncatable slices.

In the basic form of FGS enhancement layers, only inter-layer predictionis used. Therefore, FGS enhancement layers can be truncated freelywithout causing any error propagation in the decoded sequence. However,the basic form of FGS suffers from low compression efficiency. Thisissue arises because only low-quality pictures are used for interprediction references. It has therefore been proposed that FGS-enhancedpictures be used as inter prediction references. However, this may causeencoding-decoding mismatch, also referred to as drift, when some FGSdata are discarded.

One feature of a draft SVC standard is that the FGS NAL units can befreely dropped or truncated, and a feature of the SVCV standard is thatMGS NAL units can be freely dropped (but cannot be truncated) withoutaffecting the conformance of the bitstream. As discussed above, whenthose FGS or MGS data have been used for inter prediction referenceduring encoding, dropping or truncation of the data would result in amismatch between the decoded pictures in the decoder side and in theencoder side. This mismatch is also referred to as drift.

To control drift due to the dropping or truncation of FGS or MGS data,SVC applied the following solution: In a certain dependency unit, a baserepresentation (by decoding only the CGS picture with “quality_id” equalto 0 and all the dependent-on lower layer data) is stored in the decodedpicture buffer. When encoding a subsequent dependency unit with the samevalue of “dependency_id,” all of the NAL units, including FGS or MGS NALunits, use the base representation for inter prediction reference.Consequently, all drift due to dropping or truncation of FGS or MGS NALunits in an earlier access unit is stopped at this access unit. Forother dependency units with the same value of “dependency_id,” all ofthe NAL units use the decoded pictures for inter prediction reference,for high coding efficiency.

Each NAL unit includes in the NAL unit header a syntax element“use_ref_base_pic_flag.” When the value of this element is equal to 1,decoding of the NAL unit uses the base representations of the referencepictures during the inter prediction process. The syntax element“store_ref_base_pic_flag” specifies whether (when equal to 1) or not(when equal to 0) to store the base representation of the currentpicture for future pictures to use for inter prediction.

NAL units with “quality_id” greater than 0 do not contain syntaxelements related to reference picture lists construction and weightedprediction, i.e., the syntax elements “num_ref_active_1_×_minus1” (x=0or 1), the reference picture list reordering syntax table, and theweighted prediction syntax table are not present. Consequently, the MGSor FGS layers have to inherit these syntax elements from the NAL unitswith “quality_id” equal to 0 of the same dependency unit when needed.

In SVC, a reference picture list consists of either only baserepresentations (when “use_ref_base_pic_flag” is equal to 1) or onlydecoded pictures not marked as “base representation” (when“use_ref_base_pic_flag” is equal to 0), but never both at the same time.

In an H.264/AVC bit stream, coded pictures in one coded video sequenceuses the same sequence parameter set, and at any time instance duringthe decoding process, only one sequence parameter set is active. In SVC,coded pictures from different scalable layers may use different sequenceparameter sets. If different sequence parameter sets are used, then, atany time instant during the decoding process, there may be more than oneactive sequence picture parameter set. In the SVC specification, the onefor the top layer is denoted as the active sequence picture parameterset, while the rest are referred to as layer active sequence pictureparameter sets. Any given active sequence parameter set remainsunchanged throughout a coded video sequence in the layer in which theactive sequence parameter set is referred to.

As indicated earlier, MVC is an extension of H.264/AVC.

Many of the definitions, concepts, syntax structures, semantics, anddecoding processes of H.264/AVC apply also to MVC as such or withcertain generalizations or constraints. Some definitions, concepts,syntax structures, semantics, and decoding processes of MVC aredescribed in the following.

An access unit in MVC is defined to be a set of NAL units that areconsecutive in decoding order and contain exactly one primary codedpicture consisting of one or more view components. In addition to theprimary coded picture, an access unit may also contain one or moreredundant coded pictures, one auxiliary coded picture, or other NALunits not containing slices or slice data partitions of a coded picture.The decoding of an access unit results in one decoded picture consistingof one or more decoded view components, when decoding errors, bitstreamerrors or other errors which may affect the decoding do not occur. Inother words, an access unit in MVC contains the view components of theviews for one output time instance.

A view component in MVC is referred to as a coded representation of aview in a single access unit.

Inter-view prediction may be used in MVC and refers to prediction of aview component from decoded samples of different view components of thesame access unit. In MVC, inter-view prediction is realized similarly tointer prediction. For example, inter-view reference pictures are placedin the same reference picture list(s) as reference pictures for interprediction, and a reference index as well as a motion vector are codedor inferred similarly for inter-view and inter reference pictures.

An anchor picture is a coded picture in which all slices may referenceonly slices within the same access unit, i.e., inter-view prediction maybe used, but no inter prediction is used, and all following codedpictures in output order do not use inter prediction from any pictureprior to the coded picture in decoding order. Inter-view prediction maybe used for IDR view components that are part of a non-base view. A baseview in MVC is a view that has the minimum value of view order index ina coded video sequence. The base view can be decoded independently ofother views and does not use inter-view prediction. The base view can bedecoded by H.264/AVC decoders supporting only the single-view profiles,such as the Baseline Profile or the High Profile of H.264/AVC.

In the MVC standard, many of the sub-processes of the MVC decodingprocess use the respective sub-processes of the H.264/AVC standard byreplacing term “picture”, “frame”, and “field” in the sub-processspecification of the H.264/AVC standard by “view component”, “frame viewcomponent”, and “field view component”, respectively. Likewise, terms“picture”, “frame”, and “field” are often used in the following to mean“view component”, “frame view component”, and “field view component”,respectively.

As mentioned earlier, non-base views of MVC bitstreams may refer to asubset sequence parameter set NAL unit. A subset sequence parameter setfor MVC includes a base SPS data structure and a sequence parameter setMVC extension data structure. In MVC, coded pictures from differentviews may use different sequence parameter sets. An SPS in MVC(specifically the sequence parameter set MVC extension part of the SPSin MVC) can contain the view dependency information for inter-viewprediction. This may be used for example by signaling-aware mediagateways to construct the view dependency tree.

In the context of multiview video coding, view order index may bedefined as an index that indicates the decoding or bitstream order ofview components in an access unit. In MVC, the inter-view dependencyrelationships are indicated in a sequence parameter set MVC extension,which is included in a sequence parameter set. According to the MVCstandard, all sequence parameter set MVC extensions that are referred toby a coded video sequence are required to be identical. The followingexcerpt of the sequence parameter set MVC extension provides furtherdetails on the way inter-view dependency relationships are indicated inMVC.

seq_parameter_set_mvc_extension( ) { C Descriptor num_views_minus1 0ue(v) for( i = 0; i <= num_views_minus1; i++ ) view_id[ i ] 0 ue(v) for(i = 1; i <= num_views_minus1; i++ ) { num_anchor_refs_l0[ 0i ] 0 ue(v)for( j = 0; j < num_anchor_refs_l0[ i ]; j++ ) anchor_ref_l0[ i ][ j ] 0ue(v) num_anchor_refs_l1[ i ] 0 ue(v) for( j = 0; j <num_anchor_refs_l1[ i ]; j++ ) anchor_ref_l1[ i ][ j ] 0 ue(v) } for( i= 1; i <= num_views_minus1; i++ ) { num_non_anchor_refs_l0[ i ] 0 ue(v)for( j = 0; j < num_non_anchor_refs_l0[ i ]; j++ ) non_anchor_ref_l0[ i][ j ] 0 ue(v) num_non_anchor_refs_l1[ i ] 0 ue(v) for( j = 0; j <num_non_anchor_refs_l1[ i ]; j++ ) non_anchor_ref_l1[ i ][ j ] 0 ue(v) }...

In MVC decoding process, the variable VOIdx may represent the view orderindex of the view identified by view_id (which may be obtained from theMVC NAL unit header of the coded slice being decoded) and may be setequal to the value of i for which the syntax element view_id[i] includedin the referred subset sequence parameter set is equal to view_id.

The semantics of the sequence parameter set MVC extension may bespecified as follows. num_views_minus1 plus 1 specifies the maximumnumber of coded views in the coded video sequence. The actual number ofviews in the coded video sequence may be less than num_views_minus1plus 1. view_id[i] specifies the view_id of the view with VOIdx equal toi. num_anchor_refs_l0[i] specifies the number of view components forinter-view prediction in the initial reference picture list RefPicList0in decoding anchor view components with VOIdx equal to i.anchor_ref_l0[i][j] specifies the view_id of the j-th view component forinter-view prediction in the initial reference picture list RefPicList0in decoding anchor view components with VOIdx equal to i.num_anchor_refs_l1[i] specifies the number of view components forinter-view prediction in the initial reference picture list RefPicList1in decoding anchor view components with VOIdx equal to i.anchor_ref_l1[i][j] specifies the view_id of the j-th view component forinter-view prediction in the initial reference picture list RefPicList1in decoding an anchor view component with VOIdx equal to i.num_non_anchor_refs_l0[i] specifies the number of view components forinter-view prediction in the initial reference picture list RefPicList0in decoding non-anchor view components with VOIdx equal to i.non_anchor_ref_l0[i][j] specifies the view_id of the j-th view componentfor inter-view prediction in the initial reference picture listRefPicList0 in decoding non-anchor view components with VOIdx equal toi. num_non_anchor_refs_l1[i] specifies the number of view components forinter-view prediction in the initial reference picture list RefPicList1in decoding non-anchor view components with VOIdx equal to i.non_anchor_ref_l1[i][j] specifies the view_id of the j-th view componentfor inter-view prediction in the initial reference picture listRefPicList1 in decoding non-anchor view components with VOIdx equal toi. For any particular view with view_id equal to vId1 and VOIdx equal tovOIdx1 and another view with view_id equal to vId2 and VOIdx equal tovOIdx2, when vId2 is equal to the value of one ofnon_anchor_ref_l0[vOIdx1][j] for all j in the range of 0 tonum_non_anchor_refs_l0[vOIdx1], exclusive, or one ofnon_anchor_ref_l1[vOIdx1][j] for all j in the range of 0 tonum_non_anchor_refs_l1[vOIdx1], exclusive, vId2 is also required to beequal to the value of one of anchor_ref_l0[vOIdx1][j] for all j in therange of 0 to num_anchor_refs_l0[vOIdx1], exclusive, or one ofanchor_ref_l1[vOIdx1][j] for all j in the range of 0 tonum_anchor_refs_l1[vOIdx1], exclusive. The inter-view dependency fornon-anchor view components is a subset of that for anchor viewcomponents.

In MVC, an operation point may be defined as follows: An operation pointis identified by a temporal_id value representing the target temporallevel and a set of view_id values representing the target output views.One operation point is associated with a bitstream subset, whichconsists of the target output views and all other views the targetoutput views depend on, that is derived using the sub-bitstreamextraction process with tIdTarget equal to the temporal_id value andviewIdTargetList consisting of the set of view_id values as inputs. Morethan one operation point may be associated with the same bitstreamsubset. When “an operation point is decoded”, a bitstream subsetcorresponding to the operation point may be decoded and subsequently thetarget output views may be output.

In SVC and MVC, a prefix NAL unit may be defined as a NAL unit thatimmediately precedes in decoding order a VCL NAL unit for baselayer/view coded slices. The NAL unit that immediately succeeds theprefix NAL unit in decoding order may be referred to as the associatedNAL unit. The prefix NAL unit contains data associated with theassociated NAL unit, which may be considered to be part of theassociated NAL unit. The prefix NAL unit may be used to include syntaxelements that affect the decoding of the base layer/view coded slices,when SVC or MVC decoding process is in use. An H.264/AVC base layer/viewdecoder may omit the prefix NAL unit in its decoding process. A prefixNAL unit could likewise be specified for other NAL-unit structuredstreams, such as HEVC. Similarly to prefix NAL units, suffix NAL unitsmay be specified and a suffix NAL unit may apply to the NAL unitpreceding it in bitstream or decoding order.

In scalable multiview coding, the same bitstream may contain coded viewcomponents of multiple views and at least some coded view components maybe coded using quality and/or spatial scalability.

There are ongoing standardization activities for depth-enhanced videocoding where both texture views and depth views are coded.

A texture view refers to a view that represents ordinary video content,for example has been captured using an ordinary camera, and is usuallysuitable for rendering on a display. A texture view typically comprisespictures having three components, one luma component and two chromacomponents. In the following, a texture picture typically comprises allits component pictures or color components unless otherwise indicatedfor example with terms luma texture picture and chroma texture picture.

Ranging information for a particular view represents distanceinformation of a texture sample from the camera sensor, disparity orparallax information between a texture sample and a respective texturesample in another view, or similar information.

Ranging information of real-word 3D scene depends on the content and mayvary for example from 0 to infinity. Different types of representationof such ranging information can be utilized.

A depth view refers to a view that represents distance information of atexture sample from the camera sensor, disparity or parallax informationbetween a texture sample and a respective texture sample in anotherview, or similar information. A depth view may comprise depth pictures(a.k.a. depth maps) having one component, similar to the luma componentof texture views. A depth map is an image with per-pixel depthinformation or similar. For example, each sample in a depth maprepresents the distance of the respective texture sample or samples fromthe plane on which the camera lies. In other words, if the z axis isalong the shooting axis of the cameras (and hence orthogonal to theplane on which the cameras lie), a sample in a depth map represents thevalue on the z axis. The semantics of depth map values may for exampleinclude the following:

Each luma sample value in a coded depth view component represents aninverse of real-world distance (Z) value, i.e. 1/Z, normalized in thedynamic range of the luma samples, such as to the range of 0 to 255,inclusive, for 8-bit luma representation. The normalization may be donein a manner where the quantization 1/Z is uniform in terms of disparity.

Each luma sample value in a coded depth view component represents aninverse of real-world distance (Z) value, i.e. 1/Z, which is mapped tothe dynamic range of the luma samples, such as to the range of 0 to 255,inclusive, for 8-bit luma representation, using a mapping functionf(1/Z) or table, such as a piece-wise linear mapping. In other words,depth map values result in applying the function f(1/Z).

Each luma sample value in a coded depth view component represents areal-world distance (Z) value normalized in the dynamic range of theluma samples, such as to the range of 0 to 255, inclusive, for 8-bitluma representation.

Each luma sample value in a coded depth view component represents adisparity or parallax value from the present depth view to anotherindicated or derived depth view or view position.

The semantics of depth map values may be indicated in the bitstream forexample within a video parameter set syntax structure, a sequenceparameter set syntax structure, a video usability information syntaxstructure, a picture parameter set syntax structure, acamera/depth/adaptation parameter set syntax structure, a supplementalenhancement information message, or anything alike.

While phrases such as depth view, depth view component, depth pictureand depth map are used to describe various embodiments, it is to beunderstood that any semantics of depth map values may be used in variousembodiments including but not limited to the ones described above. Forexample, embodiments of the invention may be applied for depth pictureswhere sample values indicate disparity values. Phrases depth viewcomponent and depth picture may be used interchangeably and may have thesame semantics in many embodiments.

An encoding system or any other entity creating or modifying a bitstreamincluding coded depth maps may create and include information on thesemantics of depth samples and on the quantization scheme of depthsamples into the bitstream. Such information on the semantics of depthsamples and on the quantization scheme of depth samples may be forexample included in a video parameter set structure, in a sequenceparameter set structure, or in a SEI message.

Depth-enhanced video refers to texture video having one or more viewsassociated with depth video having one or more depth views. A number ofapproaches may be used for representing of depth-enhanced video,including the use of video plus depth (V+D), multiview video plus depth(MVD), and layered depth video (LDV). In the video plus depth (V+D)representation, a single view of texture and the respective view ofdepth are represented as sequences of texture picture and depthpictures, respectively. The MVD representation contains a number oftexture views and respective depth views. In the LDV representation, thetexture and depth of the central view are represented conventionally,while the texture and depth of the other views are partially representedand cover only the dis-occluded areas required for correct viewsynthesis of intermediate views.

A texture view component may be defined as a coded representation of thetexture of a view in a single access unit. A texture view component indepth-enhanced video bitstream may be coded in a manner that iscompatible with a single-view texture bitstream or a multi-view texturebitstream so that a single-view or multi-view decoder can decode thetexture views even if it has no capability to decode depth views. Forexample, an H.264/AVC decoder may decode a single texture view from adepth-enhanced H.264/AVC bitstream. A texture view component mayalternatively be coded in a manner that a decoder capable of single-viewor multi-view texture decoding, such H.264/AVC or MVC decoder, is notable to decode the texture view component for example because it usesdepth-based coding tools. A depth view component may be defined as acoded representation of the depth of a view in a single access unit. Aview component pair may be defined as a texture view component and adepth view component of the same view within the same access unit.

Depth-enhanced video may be coded in a manner where texture and depthare coded independently of each other. For example, texture views may becoded as one MVC bitstream and depth views may be coded as another MVCbitstream. Depth-enhanced video may also be coded in a manner wheretexture and depth are jointly coded. In a form of a joint coding oftexture and depth views, some decoded samples of a texture picture ordata elements for decoding of a texture picture are predicted or derivedfrom some decoded samples of a depth picture or data elements obtainedin the decoding process of a depth picture. Alternatively or inaddition, some decoded samples of a depth picture or data elements fordecoding of a depth picture are predicted or derived from some decodedsamples of a texture picture or data elements obtained in the decodingprocess of a texture picture. In another option, coded video data oftexture and coded video data of depth are not predicted from each otheror one is not coded/decoded on the basis of the other one, but codedtexture and depth view may be multiplexed into the same bitstream in theencoding and demultiplexed from the bitstream in the decoding. In yetanother option, while coded video data of texture is not predicted fromcoded video data of depth in e.g. below slice layer, some of thehigh-level coding structures of texture views and depth views may beshared or predicted from each other. For example, a slice header ofcoded depth slice may be predicted from a slice header of a codedtexture slice. Moreover, some of the parameter sets may be used by bothcoded texture views and coded depth views.

Depth-enhanced video formats enable generation of virtual views orpictures at camera positions that are not represented by any of thecoded views. Generally, any depth-image-based rendering (DIBR) algorithmmay be used for synthesizing views.

Texture views and depth views may be coded into a single bitstream wheresome of the texture views may be compatible with one or more videostandards such as H.264/AVC and/or MVC. In other words, a decoder may beable to decode some of the texture views of such a bitstream and canomit the remaining texture views and depth views.

An amendment has been specified for the H.264/AVC for depth map coding.The amendment is called MVC extension for inclusion of depth maps andmay be referred to as MVC+D. The MVC+D amendment specifies theencapsulation of texture views and depth views into the same bitstreamin a manner that the texture views remain compatible with H.264/AVC andMVC so that an MVC decoder is able to decode all texture views of anMVC+D bitstream and an H.264/AVC decoder is able to decode the basetexture view of an MVC+D bitstream. Furthermore, the VCL NAL units ofthe depth view use identical syntax, semantics, and decoding process tothose of texture views below the NAL unit header.

Development of another amendment for the H.264/AVC is ongoing at thetime of writing this patent application. This amendment, referred to as3D-AVC, requires at least one texture view to be H.264/AVC compatiblewhile further texture views may be (but need not be) MVC compatible.

An encoder that encodes one or more texture and depth views into asingle H.264/AVC and/or MVC compatible bitstream may be called as a3DV-ATM encoder. Bitstreams generated by such an encoder may be referredto as 3DV-ATM bitstreams and may be either MVC+D bitstreams or 3D-AVCbitstreams. The texture views of 3DV-ATM bitstreams are compatible withH.264/AVC (for the base view) and may be compatible with MVC (always inthe case of MVC+D bitstreams and as selected by the encoder in 3D-AVCbitstreams). The depth views of 3DV-ATM bitstreams may be compatiblewith MVC+D (always in the case of MVC+D bitstreams and as selected bythe encoder in 3D-AVC bitstreams). 3D-AVC bitstreams can include aselected number of AVC/MVC compatible texture views. Furthermore,3DV-ATM bitstream 3D-AVC bitstreams can include a selected number ofdepth views that are coded using the coding tools of the AVC/MVCstandard only. The other texture views (a.k.a. enhanced texture views)of an 3DV-ATM3D-AVC bitstream may be jointly predicted from the textureand depth views and/or the other depth views of an 3D-AVC bitstream mayuse depth coding methods not included in the AVC/MVC/MVC+D standardpresently. A decoder capable of decoding all views from 3DV-ATMbitstreams may be called as a 3DV-ATM decoder.

Inter-component prediction may be defined to comprise prediction ofsyntax element values, sample values, variable values used in thedecoding process, or anything alike from a component picture of one typeto a component picture of another type. For example, inter-componentprediction may comprise prediction of a texture view component from adepth view component, or vice versa.

In some depth-enhanced video coding and bitstreams, such as MVC+D, depthviews may refer to a differently structured sequence parameter set, suchas a subset SPS NAL unit, than the sequence parameter set for textureviews. For example, a sequence parameter set for depth views may includea sequence parameter set 3D video coding (3DVC) extension. When adifferent SPS structure is used for depth-enhanced video coding, the SPSmay be referred to as a 3D video coding (3DVC) subset SPS or a 3DVC SPS,for example. From the syntax structure point of view, a 3DVC subset SPSmay be a superset of an SPS for multiview video coding such as the MVCsubset SPS.

A depth-enhanced multiview video bitstream, such as an MVC+D bitstream,may contain two types of operation points: multiview video operationpoints (e.g. MVC operation points for MVC+D bitstreams) anddepth-enhanced operation points. Multiview video operation pointsconsisting of texture view components only may be specified by an SPSfor multiview video, for example a sequence parameter set MVC extensionincluded in an SPS referred to by one or more texture views.Depth-enhanced operation points may be specified by an SPS fordepth-enhanced video, for example a sequence parameter set MVC or 3DVCextension included in an SPS referred to by one or more depth views.

A depth-enhanced multiview video bitstream may contain or be associatedwith multiple sequence parameter sets, e.g. one for the base textureview, another one for the non-base texture views, and a third one forthe depth views. For example, an MVC+D bitstream may contain one SPS NALunit (with an SPS identifier equal to e.g. 0), one MVC subset SPS NALunit (with an SPS identifier equal to e.g. 1), and one 3DVC subset SPSNAL unit (with an SPS identifier equal to e.g. 2). The first one isdistinguished from the other two by NAL unit type, while the latter twohave different profiles, i.e., one of them indicates an MVC profile andthe other one indicates an MVC+D profile.

The coding and decoding order of texture view components and depth viewcomponents may be indicated for example in a sequence parameter set.

The depth representation information SEI message of a draft MVC+Dstandard (JCT-3V document JCT2-A1001), presented in the following, maybe regarded as an example of how information about depth representationformat may be represented. The syntax of the SEI message is as follows:

depth_represention_information( payloadSize ) { C Descriptordepth_representation_type 5 ue(v) all_views_equal_flag 5 u(1) if(all_views_equal_flag = = 0 ){ num_views_minus1 5 ue(v) numViews =num_views_minus1 + 1 }else{ numViews = 1 } for( i = 0; i < numViews; i++) { depth_representation_base_view_id[i] 5 ue(v) } if (depth_representation_type == 3 ) {depth_nonlinear_representation_num_minus1 ue(v)depth_nonlinear_representation_num =  depth_nonlinear_representation_num_minus1+1 for( i = 1; i <=depth_nonlinear_representation_num; i++ )depth_nonlinear_representation_model[ i ] ue(v) } }

The semantics of the depth representation SEI message may be specifiedas follows. The syntax elements in the depth representation informationSEI message specifies various depth representation for depth views forthe purpose of processing decoded texture and depth view componentsprior to rendering on a 3D display, such as view synthesis. It isrecommended, when present, the SEI message is associated with an IDRaccess unit for the purpose of random access. The information signaledin the SEI message applies to all the access units from the access unitthe SEI message is associated with to the next access unit, in decodingorder, containing an SEI message of the same type, exclusively, or tothe end of the coded video sequence, whichever is earlier in decodingorder.

Continuing the exemplary semantics of the depth representation SEImessage, depth_representation_type specifies the representationdefinition of luma pixels in coded frame of depth views as specified inthe table below. In the table below, disparity specifies the horizontaldisplacement between two texture views and Z value specifies thedistance from a camera.

depth_representation_type Interpretation 0 Each luma pixel value incoded frame of depth views represents an inverse of Z value normalizedin range from 1 Each luma pixel value in coded frame of depth viewsrepresents disparity normalized in range from 0 to 255 2 Each luma pixelvalue in coded frame of depth views represents Z value normalized inrange from 0 to 255 3 Each luma pixel value in coded frame of depthviews represents nonlinearly mapped disparity, normalized in range from0 to 255

Continuing the exemplary semantics of the depth representation SEImessage, all_views_equal_flag equal to 0 specifies that depthrepresentation base view may not be identical to respective values foreach view in target views. all_views_equal_flag equal to 1 specifiesthat the depth representation base views are identical to respectivevalues for all target views. depth_representaion_base_view_id[i]specifies the view identifier for the NAL unit of either base view whichthe disparity for coded depth frame of i-th view_id is derived from(depth_representation_type equal to 1 or 3) or base view which theZ-axis for the coded depth frame of i-th view_id is defined as theoptical axis of (depth_representation_type equal to 0 or 2).depth_nonlinear_representation_num_minus1+2 specifies the number ofpiecewise linear segments for mapping of depth values to a scale that isuniformly quantized in terms of disparity.depth_nonlinear_representation_model[i] specifies the piecewise linearsegments for mapping of depth values to a scale that is uniformlyquantized in terms of disparity. When depth_representation_type is equalto 3, depth view component contains nonlinearly transformed depthsamples. Variable DepthLUT [i], as specified below, is used to transformcoded depth sample values from nonlinear representation to the linearrepresentation—disparity normalized in range from 0 to 255. The shape ofthis transform is defined by means of line-segment-approximation intwo-dimensional linear-disparity-to-nonlinear-disparity space. The first(0, 0) and the last (255, 255) nodes of the curve are predefined.Positions of additional nodes are transmitted in form of deviations(depth_nonlinear_representation_model[i]) from the straight-line curve.These deviations are uniformly distributed along the whole range of 0 to255, inclusive, with spacing depending on the value of nonlinear depthrepresentation num.

Variable DepthLUT[i] for i in the range of 0 to 255, inclusive, isspecified as follows.

-   -   depth_nonlinear_representation_model[0]=0    -   depth_nonlinear_representation_model[depth_nonlinear_representation_num+1]=0    -   for(k=0; k<=depth_nonlinear_representation_num; ++k)

{    pos1 = ( 255 * k ) / (depth_nonlinear_representation_num + 1 )   dev1 = depth_nonlinear_representation_model[ k ]    pos2 = ( 255 * (k+1 ) ) /    (depth_nonlinear_representation_num + 1 ) )    dev2 =depth_nonlinear_representation_model[ k+1 ]    x1 = pos1 − dev1    y1 =pos1 + dev1    x2 = pos2 − dev2    y2 = pos2 + dev2    for ( x = max(x1, 0 ); x <= min( x2, 255 ); ++x )      DepthLUT[ x ] = Clip3( 0, 255,Round( ( ( x − x1 ) * ( y2 − y1 ) ) ÷ ( x2 − x1 ) + y1 ) ) }

In a scheme referred to as unpaired multiview video-plus-depth (MVD),there may be an unequal number of texture and depth views, and/or someof the texture views might not have a co-located depth view, and/or someof the depth views might not have a co-located texture view, some of thedepth view components might not be temporally coinciding with textureview components or vice versa, co-located texture and depth views mightcover a different spatial area, and/or there may be more than one typeof depth view components. Encoding, decoding, and/or processing ofunpaired MVD signal may be facilitated by a depth-enhanced video coding,decoding, and/or processing scheme.

Terms co-located, collocated, and overlapping may be usedinterchangeably to indicate that a certain sample or area in a textureview component represents the same physical objects or fragments of a 3Dscene as a certain co-located/collocated/overlapping sample or area in adepth view component. In some embodiments, the sampling grid of atexture view component may be the same as the sampling grid of a depthview component, i.e. one sample of a component image, such as a lumaimage, of a texture view component corresponds to one sample of a depthview component, i.e. the physical dimensions of a sample match between acomponent image, such as a luma image, of a texture view component andthe corresponding depth view component. In some embodiments, sampledimensions (twidth×theight) of a sampling grid of a component image,such as a luma image, of a texture view component may be an integermultiple of sample dimensions (dwidth×dheight) of a sampling grid of adepth view component, i.e. twidth=m×dwidth and theight=n×dheight, wherem and n are positive integers. In some embodiments, dwidth=m×twidth anddheight=n×theight, where m and n are positive integers. In someembodiments, twidth=m×dwidth and theight=n×dheight or alternativelydwidth=m×twidth and dheight=n×theight, where m and n are positive valuesand may be non-integer. In these embodiments, an interpolation schememay be used in the encoder and in the decoder and in the view synthesisprocess and other processes to derive co-located sample values betweentexture and depth. In some embodiments, the physical position of asampling grid of a component image, such as a luma image, of a textureview component may match that of the corresponding depth view and thesample dimensions of a component image, such as a luma image, of thetexture view component may be an integer multiple of sample dimensions(dwidth×dheight) of a sampling grid of the depth view component (or viceversa)—then, the texture view component and the depth view component maybe considered to be co-located and represent the same viewpoint. In someembodiments, the position of a sampling grid of a component image, suchas a luma image, of a texture view component may have an integer-sampleoffset relative to the sampling grid position of a depth view component,or vice versa. In other words, a top-left sample of a sampling grid of acomponent image, such as a luma image, of a texture view component maycorrespond to the sample at position (x, y) in the sampling grid of adepth view component, or vice versa, where x and y are non-negativeintegers in a two-dimensional Cartesian coordinate system withnon-negative values only and origo in the top-left corner. In someembodiments, the values of x and/or y may be non-integer andconsequently an interpolation scheme may be used in the encoder and inthe decoder and in the view synthesis process and other processes toderive co-located sample values between texture and depth. In someembodiments, the sampling grid of a component image, such as a lumaimage, of a texture view component may have unequal extents compared tothose of the sampling grid of a depth view component. In other words,the number of samples in horizontal and/or vertical direction in asampling grid of a component image, such as a luma image, of a textureview component may differ from the number of samples in horizontaland/or vertical direction, respectively, in a sampling grid of a depthview component and/or the physical width and/or height of a samplinggrid of a component image, such as a luma image, of a texture viewcomponent may differ from the physical width and/or height,respectively, of a sampling grid of a depth view component. In someembodiments, non-uniform and/or non-matching sample grids can beutilized for texture and/or depth component. A sample grid of depth viewcomponent is non-matching with the sample grid of a texture viewcomponent when the sampling grid of a component image, such as a lumaimage, of the texture view component is not an integer multiple ofsample dimensions (dwidth×dheight) of a sampling grid of the depth viewcomponent or the sampling grid position of a component image, such as aluma image, of the texture view component has a non-integer offsetcompared to the sampling grid position of the depth view component orthe sampling grids of the depth view component and the texture viewcomponent are not aligned/rectified. This could happen for example onpurpose to reduce redundancy of data in one of the components or due toinaccuracy of the calibration/rectification process between a depthsensor and a color image sensor.

A coded depth-enhanced video bitstream, such as an MVC+D bitstream or an3D-AVC bitstream, may be considered to include different types ofoperation points: texture video operation points, such as MVC operationpoints, texture-plus-depth operation points including both texture viewsand depth views, and depth video operation points including only depthviews. An MVC operation point comprises texture view components asspecified by the SPS MVC extension. The texture plus depth operationpoints may be paired or unpaired. In paired texture-plus-depth operationpoints, each view contains both a texture depth and a depth view (ifboth are defined in the 3DVC subset SPS by the same syntax structure asthat used in the SPS MVC extension. originally present in thebitstream). In unpaired texture-plus-depth operation points, it isspecified whether a texture view or a depth view or both are present inthe operation point for a particular view.

The coding and/or decoding order of texture view components and depthview components may determine presence of syntax elements related tointer-component prediction and allowed values of syntax elements relatedto inter-component prediction.

In the case of joint coding of texture and depth for depth-enhancedvideo, view synthesis can be utilized in the loop of the codec, thusproviding view synthesis prediction (VSP). In VSP, a prediction signal,such as a VSP reference picture, is formed using a DIBR or viewsynthesis algorithm, utilizing texture and depth information. Forexample, a synthesized picture (i.e., VSP reference picture) may beintroduced in the reference picture list in a similar way as it is donewith interview reference pictures and inter-view only referencepictures. Alternatively or in addition, a specific VSP prediction modefor certain prediction blocks may be determined by the encoder,indicated in the bitstream by the encoder, and used as concluded fromthe bitstream by the decoder.

In MVC, both inter prediction and inter-view prediction use similarmotion-compensated prediction process. Inter-view reference pictures andinter-view only reference pictures are essentially treated as long-termreference pictures in the different prediction processes. Similarly,view synthesis prediction may be realized in such a manner that it usesessentially the same motion-compensated prediction process as interprediction and inter-view prediction. To differentiate frommotion-compensated prediction taking place only within a single viewwithout any VSP, motion-compensated prediction that includes and iscapable of flexibly selecting mixing inter prediction, inter-prediction,and/or view synthesis prediction is herein referred to asmixed-direction motion-compensated prediction.

As reference picture lists in MVC and an envisioned coding scheme forMVD such as 3DV-ATM and in similar coding schemes may contain more thanone type of reference pictures, such as inter reference pictures (alsoknown as intra-view reference pictures), inter-view reference pictures,inter-view only reference pictures, and/or VSP reference pictures, aterm prediction direction may be defined to indicate the use ofintra-view reference pictures (temporal prediction), inter-viewprediction, or VSP. For example, an encoder may choose for a specificblock a reference index that points to an inter-view reference picture,thus the prediction direction of the block is inter-view.

A VSP reference picture may also be referred to as a synthetic referencecomponent, which may be defined to contain samples that may be used forview synthesis prediction. A synthetic reference component may be usedas a reference picture for view synthesis prediction but may not beoutput or displayed. A view synthesis picture may be generated for thesame camera location assuming the same camera parameters as for thepicture being coded or decoded.

A view-synthesized picture may be introduced in the reference picturelist in a similar way as is done with inter-view reference pictures.Signaling and operations with reference picture list in the case of viewsynthesis prediction may remain identical or similar to those specifiedin H.264/AVC or HEVC.

A synthesized picture resulting from VSP may be included in the initialreference picture lists List0 and List1 for example following temporaland inter-view reference frames. However, reference picture listmodification syntax (i.e., RPLR commands) may be extended to support VSPreference pictures, thus the encoder can order reference picture listsat any order, indicate the final order with RPLR commands in thebitstream, causing the decoder to reconstruct the reference picturelists having the same final order.

Processes for predicting from view synthesis reference picture, such asmotion information derivation, may remain identical or similar toprocesses specified for inter, inter-layer, and inter-view prediction ofH.264/AVC or HEVC. Alternatively or in addition, specific coding modesfor the view synthesis prediction may be specified and signaled by theencoder in the bitstream. In other words, VSP may alternatively or alsobe used in some encoding and decoding arrangements as a separate modefrom intra, inter, inter-view and other coding modes. For example, in aVSP skip/direct mode the motion vector difference (de)coding and the(de)coding of the residual prediction error for example usingtransform-based coding may also be omitted. For example, if a macroblockmay be indicated within the bitstream to be coded using a skip/directmode, it may further be indicated within the bitstream whether a VSPframe is used as a reference. Alternatively or in addition,view-synthesized reference blocks, rather than or in addition tocomplete view synthesis reference pictures, may be generated by theencoder and/or the decoder and used as prediction reference for variousprediction processes.

To enable view synthesis prediction for the coding of the currenttexture view component, the previously coded texture and depth viewcomponents of the same access unit may be used for the view synthesis.Such a view synthesis that uses the previously coded texture and depthview components of the same access unit may be referred to as a forwardview synthesis or forward-projected view synthesis, and similarly viewsynthesis prediction using such view synthesis may be referred to asforward view synthesis prediction or forward-projected view synthesisprediction.

A coding scheme for unpaired MVD may for example include one or more ofthe following aspects:

-   -   Encoding one or more indications of which ones of the input        texture and depth views are encoded, inter-view prediction        hierarchy of texture views and depth views, and/or AU view        component order into a bitstream.

As a response of a depth view required as a reference or input forprediction (such as view synthesis prediction, inter-view prediction,inter-component prediction, and/or alike) and/or for view synthesisperformed as post-processing for decoding and the depth view not inputto the encoder or determined not to be coded, performing the following:

-   -   Deriving the depth view, one or more depth view components for        the depth view, or parts of one or more depth view components        for the depth view on the basis of coded depth views and/or        coded texture views and/or reconstructed depth views and/or        reconstructed texture views or parts of them. The derivation may        be based on view synthesis or DIBR, for example.

Using the derived depth view as a reference or input for prediction(such as view synthesis prediction, inter-view prediction,inter-component prediction, and/or alike) and/or for view synthesisperformed as post-processing for decoding.

Inferring the use of one or more coding tools, modes of coding tools,and/or coding parameters for coding a texture view based on the presenceor absence of a respective coded depth view and/or the presence orabsence of a respective derived depth view. In some embodiments, when adepth view is required as a reference or input for prediction (such asview synthesis prediction, inter-view prediction, inter-componentprediction, and/or alike) but is not encoded, the encoder may

-   -   derive the depth view; or    -   infer that coding tools causing a depth view to be required as a        reference or input for prediction are turned off; or    -   select one of the above adaptively and encode the chosen option        and related parameter values, if any, as one or more indications        into the bitstream.

Forming an inter-component prediction signal or prediction block oralike from a depth view component (or, generally from one or more depthview components) to a texture view component (or, generally to one ormore texture view components) for a subset of predicted blocks in atexture view component on the basis of availability of co-locatedsamples or blocks in a depth view component. Similarly, forming aninter-component prediction signal or a prediction block or alike from atexture view component (or, generally from one or more texture viewcomponents) to a depth view component (or, generally to one or moredepth view components) for a subset of predicted blocks in a depth viewcomponent on the basis of availability of co-located samples or blocksin a texture view component.

Forming a view synthesis prediction signal or a prediction block oralike for a texture block on the basis of availability of co-locateddepth samples.

A decoding scheme for unpaired MVD may for example include one or moreof the following aspects:

-   -   Receiving and decoding one or more indications of coded texture        and depth views, inter-view prediction hierarchy of texture        views and depth views, and/or AU view component order from a        bitstream.

When a depth view required as a reference or input for prediction (suchas view synthesis prediction, inter-view prediction, inter-componentprediction, and/or alike) but not included in the received bitstream,

-   -   deriving the depth view; or    -   inferring that coding tools causing a depth view to be required        as a reference or input for prediction are turned off; or    -   selecting one of the above based on one or more indications        received and decoded from the bitstream.

Inferring the use of one or more coding tools, modes of coding tools,and/or coding parameters for decoding a texture view based on thepresence or absence of a respective coded depth view and/or the presenceor absence of a respective derived depth view.

Forming an inter-component prediction signal or prediction block oralike from a depth view component (or, generally from one or more depthview components) to a texture view component (or, generally to one ormore texture view components) for a subset of predicted blocks in atexture view component on the basis of availability of co-locatedsamples or blocks in a depth view component. Similarly, forming aninter-component prediction signal or prediction block or alike from atexture view component (or, generally from one or more texture viewcomponents) to a depth view component (or, generally to one or moredepth view components) for a subset of predicted blocks in a depth viewcomponent on the basis of availability of co-located samples or blocksin a texture view component.

Forming a view synthesis prediction signal or prediction block or alikeon the basis of availability of co-located depth samples.

When a depth view required as a reference or input for prediction forview synthesis performed as post-processing, deriving the depth view.

Determining view components that are not needed for decoding or outputon the basis of mentioned signalling and configuring the decoder toavoid decoding these unnecessary coded view components.

Video compression is commonly achieved by removing spatial, frequency,and/or temporal redundancies. Different types of prediction andquantization of transform-domain prediction residuals may be used toexploit both spatial and temporal redundancies. In addition, as codingschemes have a practical limit in the redundancy that can be removed,spatial and temporal sampling frequency as well as the bit depth ofsamples can be selected in such a manner that the subjective quality isdegraded as little as possible.

One potential way for obtaining compression improvement in stereoscopicvideo is an asymmetric stereoscopic video coding, in which there is aquality difference between two coded views. This is attributed to thewidely believed assumption of the binocular suppression theory that theHuman Visual System (HVS) fuses the stereoscopic image pair such thatthe perceived quality is close to that of the higher quality view.

Asymmetry between the two views can be achieved e.g. by one or more ofthe following methods:

-   -   Mixed-resolution (MR) stereoscopic video coding, which may also        be referred to as resolution-asymmetric stereoscopic video        coding, in which one of the views is low-pass filtered and hence        has a smaller amount of spatial details or a lower spatial        resolution. Furthermore, the low-pass filtered view may be        sampled with a coarser sampling grid, i.e., represented by fewer        pixels.    -   Mixed-resolution chroma sampling, in which the chroma pictures        of one view are represented by fewer samples than the respective        chroma pictures of the other view.

Asymmetric sample-domain quantization, in which the sample values of thetwo views are quantized with a different step size. For example, theluma samples of one view may be represented with the range of 0 to 255(i.e., 8 bits per sample) while the range may be scaled e.g. to therange of 0 to 159 for the second view. Thanks to fewer quantizationsteps, the second view can be compressed with a higher ratio compared tothe first view. Different quantization step sizes may be used for lumaand chroma samples. As a special case of asymmetric sample-domainquantization, one can refer to bit-depth-asymmetric stereoscopic videowhen the number of quantization steps in each view matches a power oftwo.

Asymmetric transform-domain quantization, in which the transformcoefficients of the two views are quantized with a different step size.As a result, one of the views has a lower fidelity and may be subject toa greater amount of visible coding artifacts, such as blocking andringing.

A combination of different encoding techniques above may also be used.

The aforementioned types of asymmetric stereoscopic video coding areillustrated in FIG. 10. The first row (12 a) presents the higher qualityview which is only transform-coded. The remaining rows (12 b-12 e)present several encoding combinations which have been investigated tocreate the lower quality view using different steps, namely,downsampling, sample domain quantization, and transform based coding. Itcan be observed from the figure that downsampling or sample-domainquantization can be applied or skipped regardless of how other steps inthe processing chain are applied. Likewise, the quantization step in thetransform-domain coding step can be selected independently of the othersteps. Thus, practical realizations of asymmetric stereoscopic videocoding may use appropriate techniques for achieving asymmetry in acombined manner as illustrated in FIG. 12 e.

In addition to the aforementioned types of asymmetric stereoscopic videocoding, mixed temporal resolution (i.e., different picture rate) betweenviews may also be used.

In multiview video coding, motion vectors of different views may bequite correlated as the views are captured from cameras that areslightly apart each other. Therefore, utilizing motion data of one viewfor coding the other view may improve the coding efficiency of amultiview video coder.

Multiview video coding may be realized in many ways. For example,multiview video coding may be realized by only introducing high levelsyntax changes to a single layer video coder without any changes belowthe macroblock (or coding tree block) layer. In this high-level onlymultiview video coder, the decoded pictures from different views may beplaced in the decoded picture buffer (DPB) of other views and treated asa regular reference picture.

Temporal motion vector prediction process may be used to exploit theredundancy of motion data between different layers. This may be done asfollows: when the base layer is upsampled the motion data of the baselayer is also mapped to the resolution of an enhancement layer. If theenhancement layer picture utilizes temporal motion vector predictionfrom the base layer picture, the corresponding motion vector predictoris originated from the mapped base layer motion field. This way thecorrelation between the motion data of different layers may be exploitedto improve the coding efficiency of a scalable video coder.

This kind of motion mapping process may be useful for mapping motionfields between layers of different resolutions, but may not work formulti-view video coding.

In an inter-view motion skip or prediction mode for multi-view videocoding correlations of motion data existing between different views maybe exploited. If this mode is enabled, motion data of the correspondingblock may be calculated using the motion information from a differentview. This calculation may involve first finding the correspondingmotion blocks in another view due to disparity, and performing apre-defined operation on the corresponding motion blocks. Due to the newmode, this approach may not be suitable for a high-level syntax onlymulti-view video coder.

It may also be possible to use motion of one view to predict motion ofanother view by establishing a correspondence between a block in oneview and a block in a reference view. This may be done by estimating adepth map, either based on already transmitted depth data or by usingtransmitted disparity vectors. Establishing the correspondence may beimplemented in a high-level syntax only coder.

Different measure may be derived from a block of depth samples cb_d,some of which are presented in the following. The depth/disparityinformation can be aggregatively presented through averagedepth/disparity values for cb_d and deviation (e.g. variance) of cb_d.The average Av(cb_d) depth/disparity value for a block of depthinformation cb_d may be computed as:

Av(cb_d)=sum(cb_d(x,y))/num_pixels  (4)

where x and y are coordinates of the pixels in cb_d, and num_pixels isnumber of pixels within cb_d, and function sum adds up all thesample/pixel values in the given block, i.e. function sum(block(x,y))computes a sum of samples values within the given block for all valuesof x and y corresponding to the horizontal and vertical extents of theblock.

The deviation Dev(cb_d) of the depth/disparity values within a block ofdepth information cb_d can be computed as:

Dev(cb_d)=sum(abs(cb_d(x,y)−Av(cb_d)))/num_pixels  (5)

where function abs returns the absolute value of the value given asinput.

The following may be used to determine if a block of depth data cb_drepresents homogenous:

If Dev(cb_d)=<T1, cb_d=homogenous data  (6)

where T1 may be an application-specific predefined threshold T1 and/ormay be indicated by the encoder in the bitstream. In other words, if thedeviation of the depth/disparity values within a block of depthinformation cb_d is less than or equal than the threshold T1, such cb_dblock can be considered as homogenous.

The similarity of two depth blocks (of the same shape and number ofpixels), cb_d and nb_d, may be compared for example in one or more ofthe following ways. One way is to compute an average pixel-wisedeviation (difference) for example as follows:

nsad(cb_d,nb_d)=sum(abs(cb_d(x,y)−nb_d(x,y)))/num_pixels  (7)

where x and y are coordinates of the pixels in cb_d and nb_d, num_pixelsis number of pixels within cb_d and functions sum and abs are definedabove. This equation may also be regarded as a sum of absolutedifferences (SAD) between the given depth blocks normalized by thenumber of pixels in the block.

In another example of a similarity or distortion metric, a sum ofsquared differences (SSD) normalized by the number of pixels may be usedas computed below:

nsse(cb_d,nb_d)=sum((cb_d(x,y)−nb_d(x,y))̂2)/num_pixels  (8)

where x and y are coordinates of the pixels in cb_d and in itsneighboring depth/disparity block (nb_d), num_pixels is number of pixelswithin cb_d, notation ̂2 indicates a power of two, and function sum isdefined above.

In another example, a sum of transformed differences (SATD) may be usedas a similarity or distortion metric. Both the current depth/disparityblock cb_d and a neighboring depth/disparity block nb_d are transformedusing for example DCT or a variant thereof, herein marked as functionTO. Let tcb_d be equal to T(cb_d) and tnb_d be equal to T(nb_d). Then,either the sum of absolute or squared differences is calculated and maybe normalized by the number of pixels/samples, num_pixels, in cb_d ornb_d, which is also equal to the number of transform coefficients intcb_d or tnb_d. In the following equation, a version of sum oftransformed differences using sum of absolute differences is given:

nsatd(cb_d,nb_d)=sum(abs(tcb_d(x,y)−tnb_d(x,y)))/num_pixels  (9)

Other distortion metrics, such as the structural similarity index(SSIM), may also be used for the derivation the similarity of two depthblocks.

The similarity or distortion metric might not performed for all samplelocation of cb_d and nb_d but only for selected sample locations, suchas the four corner samples, and/or cb_d and nb_d may be downsampledbefore performing the similarity or distortion metric computation.

Function diff(cb_d, nb_d) may be defined as follows to enable access anysimilarity or distortion metric:

diff(cb_d,nb_d)=

nsad(cb_d,nb_d), if sum of absolute differences is used

nsse(cb_d,nb_d), if sum of squared differences is used

nsatd(cb_d,nb_d), if sum of transformed absolute differences isused  (10)

Any similarity/distortion metric could be added to the definition offunction diff(cb_d, nb_d). In some embodiments, the usedsimilarity/distortion metric is pre-defined and therefore stays the samein both the encoder and the decoder. In some embodiments, the usedsimilarity/distortion metric is determined by the encoder, for exampleusing rate-distortion optimization, and encoded in the bitstream as oneor more indications. The indication(s) of the used similarity/distortionmetric may be included for example in a sequence parameter set, apicture parameter set, a slice parameter set, a picture header, a sliceheader, within a macroblock syntax structure, and/or anything alike. Insome embodiments, the indicated similarity/distortion metric may be usedin pre-determined operations in both the encoding and the decoding loop,such as depth/disparity based motion vector prediction. In someembodiments, the decoding processes for which the indicatedsimilarity/distortion metric is indicated are also indicated in thebitstream for example in a sequence parameter set, a picture parameterset, a slice parameter set, a picture header, a slice header, within amacroblock syntax structure, or anything alike. In some embodiments, itis possible to have more than one pair of indications for thedepth/disparity metric and the decoding processes the metric is appliedto in a the bitstream having the same persistence for the decodingprocess, i.e. applicable to decoding of the same access units. Theencoder may select which similarity/distortion metric is used for eachparticular decoding process where a similarity/distortion basedselection or other processing is used, such as depth/disparity basedmotion vector prediction, and encode respective indications of theselected disparity/distortion metrics and to which decoding processesthey apply to into the bitstream.

When the similarity of disparity blocks is compared, the viewpoints ofthe blocks may be normalized, e.g. so that the disparity values arescaled to result from the same camera separation in both comparedblocks.

A scalable video coding and/or decoding scheme may use multi-loop codingand/or decoding, which may be characterized as follows. In theencoding/decoding, a base layer picture may be reconstructed/decoded tobe used as a motion-compensation reference picture for subsequentpictures, in coding/decoding order, within the same layer or as areference for inter-layer (or inter-view or inter-component) prediction.The reconstructed/decoded base layer picture may be stored in the DPB.An enhancement layer picture may likewise be reconstructed/decoded to beused as a motion-compensation reference picture for subsequent pictures,in coding/decoding order, within the same layer or as reference forinter-layer (or inter-view or inter-component) prediction for higherenhancement layers, if any. In addition to reconstructed/decoded samplevalues, syntax element values of the base/reference layer or variablesderived from the syntax element values of the base/reference layer maybe used in the inter-layer/inter-component/inter-view prediction.

A scalable video encoder for quality scalability (also known asSignal-to-Noise or SNR) and/or spatial scalability may be implemented asfollows. For a base layer, a conventional non-scalable video encoder anddecoder may be used. The reconstructed/decoded pictures of the baselayer are included in the reference picture buffer and/or referencepicture lists for an enhancement layer. In case of spatial scalability,the reconstructed/decoded base-layer picture may be upsampled prior toits insertion into the reference picture lists for an enhancement-layerpicture. The base layer decoded pictures may be inserted into areference picture list(s) for coding/decoding of an enhancement layerpicture similarly to the decoded reference pictures of the enhancementlayer. Consequently, the encoder may choose a base-layer referencepicture as an inter prediction reference and indicate its use with areference picture index in the coded bitstream. The decoder decodes fromthe bitstream, for example from a reference picture index, that abase-layer picture is used as an inter prediction reference for theenhancement layer. When a decoded base-layer picture is used as theprediction reference for an enhancement layer, it is referred to as aninter-layer reference picture.

While the previous paragraph described a scalable video codec with twoscalability layers with an enhancement layer and a base layer, it needsto be understood that the description can be generalized to any twolayers in a scalability hierarchy with more than two layers. In thiscase, a second enhancement layer may depend on a first enhancement layerin encoding and/or decoding processes, and the first enhancement layermay therefore be regarded as the base layer for the encoding and/ordecoding of the second enhancement layer. Furthermore, it needs to beunderstood that there may be inter-layer reference pictures from morethan one layer in a reference picture buffer or reference picture listsof an enhancement layer, and each of these inter-layer referencepictures may be considered to reside in a base layer or a referencelayer for the enhancement layer being encoded and/or decoded.

In scalable multiview coding, the same bitstream may contain coded viewcomponents of multiple views and at least some coded view components maybe coded using quality and/or spatial scalability.

Work is ongoing to specify scalable and multiview extensions to the HEVCstandard. The multiview extension of HEVC, referred to as MV-HEVC, issimilar to the MVC extension of H.264/AVC. Similarly to MVC, in MV-HEVC,inter-view reference pictures can be included in the reference picturelist(s) of the current picture being coded or decoded. The scalableextension of HEVC, referred to as SHVC, is planned to be specified sothat it uses multi-loop decoding operation (unlike the SVC extension ofH.264/AVC). Currently, two designs to realize scalability areinvestigated for SHVC. One is reference index based, where aninter-layer reference picture can be included in a one or more referencepicture lists of the current picture being coded or decoded (asdescribed above). Another may be referred to as IntraBL or TextureRL,where a specific coding mode, e.g. in CU level, is used for usingdecoded/reconstructed sample values of a reference layer picture forprediction in an enhancement layer picture. The SHVC development hasconcentrated on development of spatial and coarse grain qualityscalability.

It is possible to use many of the same syntax structures, semantics, anddecoding processes for MV-HEVC and reference-index-based SHVC.Furthermore, it is possible to use the same syntax structures,semantics, and decoding processes for depth coding too. Hereafter, termscalable multiview extension of HEVC (SMV-HEVC) is used to refer to acoding process, a decoding process, syntax, and semantics where largelythe same (de)coding tools are used regardless of the scalability typeand where the reference index based approach without changes in thesyntax, semantics, or decoding process below the slice header is used.SMV-HEVC might not be limited to multiview, spatial, and coarse grainquality scalability but may also support other types of scalability,such as depth-enhanced video.

For the enhancement layer coding, the same concepts and coding tools ofHEVC may be used in SHVC, MV-HEVC, and/or SMV-HEVC. However, theadditional inter-layer prediction tools, which employ already coded data(including reconstructed picture samples and motion parameters a.k.amotion information) in reference layer for efficiently coding anenhancement layer, may be integrated to SHVC, MV-HEVC, and/or SMV-HEVCcodec.

In MV-HEVC, SMV-HEVC, and reference index based SHVC solution, the blocklevel syntax and decoding process are not changed for supportinginter-layer texture prediction. Only the high-level syntax has beenmodified (compared to that of HEVC) so that reconstructed pictures(upsampled if necessary) from a reference layer of the same access unitcan be used as the reference pictures for coding the current enhancementlayer picture. The inter-layer reference pictures as well as thetemporal reference pictures are included in the reference picture lists.The signalled reference picture index is used to indicate whether thecurrent Prediction Unit (PU) is predicted from a temporal referencepicture or an inter-layer reference picture. The use of this feature maybe controlled by the encoder and indicated in the bitstream for examplein a video parameter set, a sequence parameter set, a picture parameter,and/or a slice header. The indication(s) may be specific to anenhancement layer, a reference layer, a pair of an enhancement layer anda reference layer, specific TemporalId values, specific picture types(e.g. RAP pictures), specific slice types (e.g. P and B slices but not Islices), pictures of a specific POC value, and/or specific access units,for example. The scope an+d/or persistence of the indication(s) may beindicated along with the indication(s) themselves and/or may beinferred.

The reference list(s) in MV-HEVC, SMV-HEVC, and a reference index basedSHVC solution may be initialized using a specific process in which theinter-layer reference picture(s), if any, may be included in the initialreference picture list(s). are constructed as follows. For example, thetemporal references may be firstly added into the reference lists (L0,L1) in the same manner as the reference list construction in HEVC. Afterthat, the inter-layer references may be added after the temporalreferences. The inter-layer reference pictures may be for exampleconcluded from the layer dependency information, such as theRefLayerId[i] variable derived from the VPS extension as describedabove. The inter-layer reference pictures may be added to the initialreference picture list L0 if the current enhancement-layer slice is aP-Slice, and may be added to both initial reference picture lists L0 andL1 if the current enhancement-layer slice is a B-Slice. The inter-layerreference pictures may be added to the reference picture lists in aspecific order, which can but need not be the same for both referencepicture lists. For example, an opposite order of adding inter-layerreference pictures into the initial reference picture list 1 may be usedcompared to that of the initial reference picture list 0. For example,inter-layer reference pictures may be inserted into the initialreference picture 0 in an ascending order of nuh_layer_id, while anopposite order may be used to initialize the initial reference picturelist 1.

In the coding and/or decoding process, the inter-layer referencepictures may be treated as long term reference pictures.

In SMV-HEVC and a reference index based SHVC solution, inter-layermotion parameter prediction may be performed by setting the inter-layerreference picture as the collocated reference picture for TMVPderivation. A motion field mapping process between two layers may beperformed for example to avoid block level decoding process modificationin TMVP derivation. A motion field mapping could also be performed formultiview coding, but a present draft of MV-HEVC does not include such aprocess. The use of the motion field mapping feature may be controlledby the encoder and indicated in the bitstream for example in a videoparameter set, a sequence parameter set, a picture parameter, and/or aslice header. The indication(s) may be specific to an enhancement layer,a reference layer, a pair of an enhancement layer and a reference layer,specific TemporalId values, specific picture types (e.g. RAP pictures),specific slice types (e.g. P and B slices but not I slices), pictures ofa specific POC value, and/or specific access units, for example. Thescope and/or persistence of the indication(s) may be indicated alongwith the indication(s) themselves and/or may be inferred.

In a motion field mapping process for spatial scalability, the motionfield of the upsampled inter-layer reference picture is attained basedon the motion field of the respective reference layer picture. Themotion parameters (which may e.g. include a horizontal and/or verticalmotion vector value and a reference index) and/or a prediction mode foreach block of the upsampled inter-layer reference picture may be derivedfrom the corresponding motion parameters and/or prediction mode of thecollocated block in the reference layer picture. The block size used forthe derivation of the motion parameters and/or prediction mode in theupsampled inter-layer reference picture may be for example 16×16. The16×16 block size is the same as in HEVC TMVP derivation process wherecompressed motion field of reference picture is used.

The TMVP process of HEVC is limited to one target picture per slice inthe merge mode and one collocated picture (per slice). When applying thereference index based scalability on top of HEVC, the TMVP process ofHEVC has limited applicability as explained in the following in the caseof the merge mode. In the example, the target reference picture (withindex 0 in the reference picture list) is a short-term referencepicture. The motion vector in the collocated PU, if referring to ashort-term (ST) reference picture, is scaled to form a merge candidateof the current PU (PU0), as shown in the FIG. 9a , wherein MV0 is scaledto MV0′ during the merge mode process. However, if the collocated PU hasa motion vector (MV1) referring to an inter-view reference picture,marked as long-term, the motion vector is not used to predict thecurrent PU (PU1).

There might be a significant amount of collocated PUs (in the collocatedpicture) which contain motion vectors referring to an inter-viewreference picture while the target reference index (being equal to 0)indicates a short-term reference picture. Therefore, disablingprediction from those motion vectors makes the merge mode lessefficient. There have been proposals to overcome this issue, some ofwhich are explained in the following paragraphs.

An additional target reference index may be indicated by the encoder inthe bitstream and decoded by the decoder from the bitstream and/orinferred by the encoder and/or the decoder. As shown in the FIG. 9b ,MV1 of the co-located block of PU1 can be used to form a disparitymotion vector merging candidate. In general, when the reference indexequal to 0 represents a short-term reference picture, the additionaltarget reference index is used to represent a long-term referencepicture. When the reference index equal to 0 represents a long-termreference picture, the additional target reference index is used torepresent a short-term reference picture.

The methods to indicate or infer the additional reference index includebut are not limited to the following:

Indication the additional target reference index in the bitstream, forexample within the slice segment header syntax structure.

Deriving the changed target reference index to be equal to the smallestreference index which has a different marking (as used as short-term orlong-term reference) from that of reference index 0.

In the case the co-located PU points to a reference picture having adifferent layer identifier (equal to layerA) than that for referenceindex 0, deriving the changed target reference index to be equal to thesmallest reference index that has layer identifier equal to layerA.

In the merge mode process the default target picture (with referenceindex 0) is used when its marking as short-term or long-term referencepicture is the same as that of the reference picture of the collocatedblock. Otherwise (i.e., when the marking of the reference picturecorresponding to the additional reference index as short-term orlong-term reference picture is the same as that of the reference pictureof the collocated block), the target picture identified by theadditional reference index is used.

In a textureRL based SHVC solution, the inter-layer texture predictionmay be performed at CU level for which a new prediction mode, named astextureRL mode, is introduced. The collocated upsampled base layer blockis used as the prediction for the enhancement layer CU coded intextureRL mode. For an input CU of the enhancement layer encoder, the CUmode may be determined among intra, inter and textureRL modes, forexample. The use of the textureRL feature may be controlled by theencoder and indicated in the bitstream for example in a video parameterset, a sequence parameter set, a picture parameter, and/or a sliceheader. The indication(s) may be specific to an enhancement layer, areference layer, a pair of an enhancement layer and a reference layer,specific TemporalId values, specific picture types (e.g. RAP pictures),specific slice types (e.g. P and B slices but not I slices), pictures ofa specific POC value, and/or specific access units, for example. Thescope and/or persistence of the indication(s) may be indicated alongwith the indication(s) themselves and/or may be inferred. Furthermore,the textureRL may be selected by the encoder at CU level and may beindicated in the bitstream per each CU for example using a CU level flag(texture_rl_flag) which may be entropy-coded e.g. using context adaptivearithmetic coding (e.g. CABAC).

The residue of textureRL predicted CU may be coded as follows. Thetransform process of textureRL predicted CU may be the same as that forthe intra predicted CU, where a discrete sine transform (DST) is appliedto TU of luma component having 4×4 size and a discrete cosine transform(DCT) is applied to the other type of TUs. Transform coefficient codingof a textureRL-predicted CU may be the same to that of inter predictedCU, where no_residue_flag may be used to indicate whether thecoefficients of the whole CU are skipped.

In a textureRL based SHVC solution, in addition to spatially andtemporally neighboring PUs, the motion parameters of the collocatedreference-layer block may also used to form the merge candidate list.The base layer merge candidate may be derived at a location collocatedto the central position of the current PU and may be inserted in aparticular location of the merge list, such as as the first candidate inmerge list. In the case of spatial scalability, the reference-layermotion vector may be scaled according to the spatial resolution ratiobetween the two layers. The pruning (duplicated candidates check) may beperformed for each spatially neighboring candidate with collocated baselayer candidate. For the collocated base layer merge candidate andspatial merge candidate derivation, a certain maximum number of mergecandidates may be used; for example four merge candidates may beselected among candidates that are located in six different positions.The temporal merge candidate may be derived in the same manner as donefor HEVC merge list. When the number of candidates does not reach tomaximum number of merge candidates (which may be determined by theencoder and may be indicated in the bitstream and may be assigned to thevariable MaxNumMergeCand), the additional candidates, including combinedbi-predictive candidates and zero merge candidates, may be generated andadded at the end of the merge list, similarly or identically to HEVCmerge list construction.

In some coding and/or decoding arrangements, a reference index basedscalability and a block-level scalability approach, such a textureRLbased approach, may be combined. For example, multiview-video-plus-depthcoding and/or decoding may be performed as follows. A textureRL approachmay be used between the components of the same view. For example, adepth view component may be inter-layer predicted using a textureRLapproach from a texture view component of the same view. A referenceindex based approach may be used used for inter-view prediction, and insome embodiments inter-view prediction may be applied only between viewcomponents of the same component type.

Work is also ongoing to specify depth-enhanced video coding extensionsto the HEVC standard, which may be referred to as 3D-HEVC, in whichtexture views and depth views may be coded into a single bitstream wheresome of the texture views may be compatible with HEVC. In other words,an HEVC decoder may be able to decode some of the texture views of sucha bitstream and can omit the remaining texture views and depth views.

Other types of scalability and scalable video coding include bit-depthscalability, where base layer pictures are coded at lower bit-depth(e.g. 8 bits) per luma and/or chroma sample than enhancement layerpictures (e.g. 10 or 12 bits), chroma format scalability, whereenhancement layer pictures provide higher fidelity and/or higher spatialresolution in chroma (e.g. coded in 4:4:4 chroma format) than base layerpictures (e.g. 4:2:0 format), and color gamut scalability, where theenhancement layer pictures have a richer/broader color representationrange than that of the base layer pictures—for example the enhancementlayer may have UHDTV (ITU-R BT.2020) color gamut and the base layer mayhave the ITU-R BT.709 color gamut. Any number of such other types ofscalability may be realized for example with a reference index basedapproach or a block-based approach e.g. as described above.

An access unit and a coded picture may be defined for example in one ofthe following ways in various HEVC extensions:

-   -   A coded picture may be defined as a coded representation of a        picture comprising VCL NAL units with a particular value of        nuh_layer_id and containing all coding tree units of the        picture. An access unit may be defined as set of NAL units that        are associated with each other according to a specified        classification rule, are consecutive in decoding order, and        contain exactly one coded picture.    -   A coded picture may be defined a coded representation of a        picture comprising VCL NAL units with a particular value of        nuh_layer_id and containing all coding tree units of the        picture. An access unit may be defined to comprise a coded        picture with nuh_layer_id equal to 0 and zero or more coded        picture pictures with non-zero nuh_layer_id.    -   A coded picture may be defined as a coded picture to comprise        VCL NAL units of nuh_layer_id equal to 0 (only), a layer picture        may be defined to comprise VCL NAL units of a particular        non-zero nuh_layer_id. An access unit may be defined to comprise        a coded picture and zero or more layer pictures.

The constraints on NAL unit order may need to be specified usingdifferent phrasing depending on which option to define an access unitand a coded picture is used. Furthermore, the hypothetical referencedecoder (HRD) may need to be specified using different phrasingdepending on which option to define an access unit and a coded pictureis used. It is anyhow possible to specify identical NAL unit orderconstraints and HRD operation for all options. Moreover, a majority ofdecoding processes is specified for coded pictures and parts thereof(e.g. coded slices) and hence the decision on which option to define anaccess unit and a coded picture have only a small or a non-existingimpact on the way the decoding processes are specified. In someembodiments, the first option above may be used but it should beunderstood that some embodiments may be similarly described using theother definitions.

Assuming the first option to define an access unit and a coded picture,a coded video sequence may be defined as a sequence of access units thatconsists, in decoding order, of a CRA access unit with nuh_layer_idequal to 0 that is the first access unit in the bitstream, an IDR accessunit with nuh_layer_id equal to 0 or a BLA access unit with nuh_layer_idequal to 0, followed by zero or more access units none of which is anIDR access unit with nuh_layer_id equal to 0 nor a BLA access unit withnuh_layer_id equal to 0 up to but not including any subsequent IDR orBLA access unit with nuh_layer_id equal to 0.

Term temporal instant or time instant or time entity may be defined torepresent a same capturing time or output time or output order. Forexample, if a first view component of a first view is captured at thesame time as a second view component in a second view, these two viewcomponents may be considered to be of the same time instant. An accessunit may be defined to contain pictures (or view components) of the sametime instant, and hence in this case pictures residing in an access unitmay be considered to be of the same time instant. Pictures of the sametime instant may be indicated (e.g. by the encoder) using multiple meansand may be identified (e.g. by the decoding) using multiple means, suchas a picture order count (POC) value or a timestamp (e.g. an outputtimestamp).

Many video encoders utilize the Lagrangian cost function to findrate-distortion optimal coding modes, for example the desired macroblockmode and associated motion vectors. This type of cost function uses aweighting factor or X. to tie together the exact or estimated imagedistortion due to lossy coding methods and the exact or estimated amountof information required to represent the pixel/sample values in an imagearea. The Lagrangian cost function may be represented by the equation:

C=D+λR  (11)

where C is the Lagrangian cost to be minimised, D is the imagedistortion (for example, the mean-squared error between the pixel/samplevalues in original image block and in coded image block) with the modeand motion vectors currently considered, λ is a Lagrangian coefficientand R is the number of bits needed to represent the required data toreconstruct the image block in the decoder (including the amount of datato represent the candidate motion vectors).

In the following, the term layer is used in context of any type ofscalability, including view scalability and depth enhancements. Anenhancement layer refers to any type of an enhancement, such as SNR,spatial, multiview, depth, bit-depth, chroma format, and/or color gamutenhancement. A base layer also refers to any type of a base operationpoint, such as a base view, a base layer for SNR/spatial scalability, ora texture base view for depth-enhanced video coding.

There are ongoing standardization activities to specify a multiviewextension of HEVC (which may be referred to as MV-HEVC), adepth-enhanced multiview extension of HEVC (which may be referred to as3D-HEVC), and a scalable extension of HEVC (which may be referred to asSHVC). A multi-loop decoding operation has been envisioned to be used inall these specifications.

In scalable video coding schemes utilizing multi-loop (de)coding,decoded reference pictures for each (de)coded layer may be maintained ina decoded picture buffer (DPB). The memory consumption for DPB maytherefore be significantly higher than that for scalable video codingschemes with single-loop (de)coding operation. However, multi-loop(de)coding may have other advantages, such as relatively few additionalparts compared to single-layer coding.

In scalable video coding with multi-loop decoding, enhanced layers maybe predicted from pictures that had been already decoded in the base(reference) layer. Such pictures may be stored in the DPB of base layerand may be marked as used for reference. In certain circumstances, apicture marked as used for reference may be stored in fast memory, inorder to provide fast random access to its samples, and may remainstored after the picture is supposed to be displayed in order to be usedas reference for prediction. This imposes requirements on memoryorganization. In order to relax such memory requirements, a conventionaldesign in multi-loop multilayer video coding schemes (such as MVC)assumes restricted utilization of inter-layer predictions.Inter-layer/inter-view prediction for enhanced view is allowed from adecoded picture of the base view located at the same access unit, inother words, representing the scene at the same time entity. In suchdesigns, the number of reference pictures available for predictingenhanced views is increased by 1 for each reference view.

It has been proposed that in scalable video coding with multi-loop(de)coding operation pictures marked as used for reference need notoriginate from the same access units in all layers. For example, asmaller number of reference pictures may be maintained in an enhancementlayer compared to the base layer. In some embodiments a temporalinter-layer prediction, which may also be referred to as a diagonalinter-layer prediction or diagonal prediction, can be used to improvecompression efficiency in such coding scenarios.

Another, complementary way of categorizing different types of predictionis to consider across which domains or scalability types the predictioncrosses. This categorization may lead into one or more of the followingtypes of prediction, which may also sometimes be referred to asprediction directions:

Temporal prediction e.g. of sample values or motion vectors from anearlier picture usually of the same scalability layer, view andcomponent type (texture or depth).

Inter-view prediction (which may be also referred to as cross-viewprediction) referring to prediction taking place between view componentsusually of the same time instant or access unit and the same componenttype.

Inter-layer prediction referring to prediction taking place betweenlayers usually of the same time instant, of the same component type, andof the same view.

Inter-component prediction may be defined to comprise prediction ofsyntax element values, sample values, variable values used in thedecoding process, or anything alike from a component picture of one typeto a component picture of another type. For example, inter-componentprediction may comprise prediction of a texture view component from adepth view component, or vice versa.

Prediction approaches using image information from a previously codedimage can also be called as inter prediction methods. Inter predictionmay sometimes be considered to only include motion-compensated temporalprediction, while it may sometimes be considered to include all types ofprediction where a reconstructed/decoded block of samples is used asprediction source, therefore including conventional inter-viewprediction for example. Inter prediction may be considered to compriseonly sample prediction but it may alternatively be considered tocomprise both sample and syntax prediction. As a result of syntax andsample prediction, a predicted block of pixels of samples may beobtained.

If the prediction, such as predicted variable values and/or predictionblocks, is not refined by the encoder using any form of prediction erroror residual coding, prediction may be referred to as inheritance. Forexample, in the merge mode of HEVC, the prediction motion information isnot refined e.g. by (de)coding motion vector differences, and hence themerge mode may be considered as an example of motion informationinheritance.

A coded video sequence (CVS) in scalable extensions of HEVC may bespecified as follows: A coded video sequence is a sequence of accessunits that consists, in decoding order, of an IRAP access unitcontaining an IRAP picture having nuh_layer_id equal to 0 andNoRaslOutputFlag equal to 1, followed by zero or more access units thatare not IRAP access units containing an IRAP picture having nuh_layer_idequal to 0 and NoRaslOutputFlag equal to 1, including all subsequentaccess units up to but not including any subsequent access unit that isan IRAP access unit containing an IRAP picture having nuh_layer_id equalto 0 and NoRaslOutputFlag equal to 1.

Scalable bitstreams with IRAP pictures or similar that are not alignedacross layers may be used, for example more frequent RAP pictures can beused in the base layer, where they have a smaller size due to a smallerspatial resolution. A process or mechanism for layer-wise start-up ofthe decoding may be included in a video decoding scheme. Decoders mayhence start decoding of a bitstream when a base layer contains an IRAPpicture and step-wise start decoding other layers when they contain IRAPpictures. In other words, in a layer-wise start-up of the decodingprocess, decoders progressively increase the number of decoded layers(where layers may represent an enhancement in spatial resolution,quality level, views, additional components such as depth, or acombination) as subsequent pictures from additional enhancement layersare decoded in the decoding process. The progressive increase of thenumber of decoded layers may be perceived for example as a progressiveimprovement of picture quality (in case of quality and spatialscalability).

A layer-wise start-up mechanism may generate unavailable pictures forthe reference pictures of the first picture in decoding order in aparticular enhancement layer. Alternatively, a decoder may omit thedecoding of pictures preceding the IRAP picture from which the decodingof a layer can be started. These pictures that may be omitted may bespecifically labeled by the encoder or another entity within thebitstream. For example, one or more specific NAL unit types may be usedfor them. These pictures may be referred to as cross-layer random accessskip (CL-RAS) pictures.

A layer-wise start-up mechanism may start the output of enhancementlayer pictures from an IRAP picture in that enhancement layer, when allreference layers of that enhancement layer have been initializedsimilarly with an IRAP picture in the reference layers. In other words,any pictures (within the same layer) preceding such an IRAP picture inoutput order might not be output from the decoder and/or might not bedisplayed. In some cases, decodable leading pictures associated withsuch an IRAP picture may be output while other pictures preceding suchan IRAP picture might not be output.

A layer-wise start-up mechanism may be initiated in one or more of thefollowing cases:

-   -   At the beginning of a bitstream.    -   At the beginning of each coded video sequence.    -   At the beginning of a coded video sequence, when specifically        controlled, e.g. when a decoding process is started or        re-started e.g. as response to tuning into a broadcast or        seeking to a position in a file or stream.    -   At an IRAP picture (or similar) in a base layer.

If a layer-wise start-up mechanism is invoked for each coded videosequence, even if the first access unit contained non-IRAP pictures theycannot use inter prediction from reference pictures in the previouscoded video sequence. Hence, it might not be desirable to invoke alayer-wise start-up mechanism unconditionally for each coded videosequence.

Concatenation of coded video data, which may also be referred to assplicing, may occur, for example coded video sequences are concatenatedinto a bitstream that is broadcast or streamed or stored in a massmemory. For example, coded video sequences representing commercials oradvertisements may be concatenated with movies or other “primary”content.

As said above, scalable video bitstreams might contain IRAP picturesthat are not aligned across layers. It may, however, be convenient toenable concatenation of a coded video sequence that contains an IRAPpicture in the base layer in its first access unit but not necessarilyin all layers. A second coded video sequence that is spliced after afirst coded video sequence should trigger a layer-wise decoding start-upprocess. That is because the first access unit of said second codedvideo sequence might not contain an IRAP picture in all its layers andhence some reference pictures for the non-IRAP pictures in that accessunit may not be available (in the concatenated bitstream) and cannottherefore be decoded. The entity concatenating the coded videosequences, hereafter referred to as the splicer, should therefore modifythe first access unit of the second coded video sequence such that ittriggers a layer-wise start-up process in decoder(s). Embodiments forsuch splicer operation, required signal means or indications within abitstream as well as decoder operation for decoding indications andtriggering a layer-wise start-up operation are presented below.

In some embodiments, a BLA picture in the base layer may trigger alayer-wise start-up process. Hence, a splicer may convert the base layerpicture in the first access unit of said second coded video sequenceinto a BLA picture. However, in HEVC and potentially in other codingschemes, the syntax of an IDR picture and a BLA picture may differ,particularly the syntax of the slice header may differ in IDR and BLApictures. Hence, a splicer would need to rewrite the slice header if thebase layer picture in the first access unit of said second coded videosequence is an IDR picture.

It is noted that it may be undesirable to trigger a layer-wise start-upprocess in decoders always when there is an IDR picture in the baselayer. That is because non-aligned IRAP pictures may be used inconventionally coded bitstreams (when no splicing has taken place).

In some embodiments specific indication(s) are therefore provided in thebitstream syntax to indicate triggering of a layer-wise start-upprocess. These indication(s) may be generated by encoders or splicersand may be obeyed by decoders. In some embodiments, these indication(s)may be used for particular picture type(s) or NAL unit type(s) only,such as only for IDR pictures, while in other embodiments theseindication(s) may be used for any picture type(s).

Said indication(s) may for example reside in one or more of thefollowing syntax structures:

-   -   NAL unit header    -   Slice header    -   Slice segment header    -   Picture parameter set    -   Group of slices parameter set or similar    -   Picture header    -   Access unit delimiter    -   Picture delimiter    -   Prefix NAL unit    -   Suffix NAL unit

In some embodiments, said indication(s) are separate from an indicationof the picture type, such as the nal_unit_type indicated within the NALunit header structure. Hence, said indication(s) may be considered toindicate one or more cross-layer impacts while the picture type may beconsidered to indicate intra-layer impacts. Said indication(s) may applywith certain picture type(s), which may be indicated by the NAL unitheader. For example, said indication(s) may apply with IRAP picturetypes or a subset thereof. In some embodiments, said indication(s) applywith IDR picture type(s) only.

In some embodiments, a new picture type or types, which may be referredto as cross-layer instantaneous decoding refresh (CL-IDR) pictures andwhich may be indicated by the NAL unit header, may be used. The CL-IDRpictures may be categorized as IRAP pictures (or alike). There may befor example two CL-IDR picture types, CL_IDR_W_RADL and CL_IDR_N_LP,where CL_IDR_W_RADL may be associated with RADL pictures and CL_IDR_N_LPhas no leading pictures.

If said indication(s) reside in a slice header and/or in a slice segmentheader, they may use for example previously reserved bits. For example,a slice segment header syntax for a single-layer coding scheme mayinclude reserved syntax elements, which may be used for example tointroduce syntax elements that do not change the decoding of the baselayer but may change the operation of a multi-layer decoding process. Anexample embodiment using HEVC syntax is provided in the following.

In HEVC, the syntax of the slice segment header is the following:

slice_segment_header( ) { Descriptor  first_slice_segment_in_pic_flagu(1)  if( nal_unit_type >= BLA_W_LP && nal_unit_type <= RSV_IRAP_VCL23 )  no_output_of_prior_pics_flag u(1)  slice_pic_parameter_set_id ue(v) if( !first_slice_segment_in_pic_flag ) {   if(dependent_slice_segments_enabled_flag )    dependent_slice_segment_flagu(1)   slice_segment_address u(v)  }  if( !dependent_slice_segment_flag) {   for( i = 0; i < num_extra_slice_header_bits; i++ )   slice_reserved_flag[ i ] u(1)   slice_type ue(v)   if(output_flag_present_flag )    pic_output_flag u(1)   if(separate_colour_plane_flag = = 1 )    colour_plane_id u(2)   if(nal_unit_type != IDR_W_RADL && nal_unit_type != IDR_N_LP ) {   slice_pic_order_cnt_lsb u(v)    short_term_ref_pic_set_sps_flag u(1)   if( !short_term_ref_pic_set_sps_flag )     short_term_ref_pic_set(num_short_term_ref_pic_sets )    else if( num_short_term_ref_pic_sets >1 )     short_term_ref_pic_set_idx u(v)    if(long_term_ref_pics_present_flag ) {     if( num_long_term_ref_pics_sps >0 )      num_long_term_sps ue(v)     num_long_term_pics ue(v)     for( i= 0; i < num_long_term_sps + num_long_term_pics; i++ ) {      if( i <num_long_term_sps ) {       if( num_long_term_ref_pics_sps > 1 )       lt_idx_sps[ i ] u(v)      } else {        poc_lsb_lt[ i ] u(v)       used_by_curr_pic_lt_flag[ i ] u(1)       }      delta_poc_msb_present_flag[ i ] u(1)       if(delta_poc_msb_present_flag[ i ] )        delta_poc_msb_cycle_lt[ i ]ue(v)      }     }     if( sps_temporal_mvp_enabled_flag )     slice_temporal_mvp_enabled_flag u(1)    }    if(sample_adaptive_offset_enabled_flag ) {     slice_sao_luma_flag u(1)    slice_sao_chroma_flag u(1)    }    if( slice_type = = P ||slice_type = = B ) {     num_ref_idx_active_override_flag u(1)     if(num_ref_idx_active_override_flag ) {      num_ref_idx_10_active_minus1ue(v)      if( slice_type = = B )       num_ref_idx_l1_active_minus1ue(v)     }     if( lists_modification_present_flag && NumPocTotalCurr >1 )      ref_pic_lists_modification( )     if( slice_type = = B )     mvd_l1_zero_flag u(1)     if( cabac_init_present_flag )     cabac_init_flag u(1)     if( slice_temporal_mvp_enabled_flag ) {     if( slice_type = = B )      collocated_from_l0_flag u(1)     if( (collocated_from_l0_flag && num_ref_idx_l0_active_minus1 > 0 ) ||      (!collocated_from_l0_flag && num_ref_idx_l1 active_minus1 > 0 ) )     collocated_ref_idx ue(v)     }     if( ( weighted_pred_flag &&slice_type = = P ) ||      ( weighted_bipred_flag && slice_type = = B ))      pred_weight_table( )     five_minus_max_num_merge_cand ue(v)    }   slice_qp_delta se(v)    if( pps_slice_chroma_qp_offsets_present_flag) {     slice_cb_qp_offset se(v)     slice_cr_qp_offset se(v)    }   if( deblocking_filter_override_enabled_flag )    deblocking_filter_override_flag u(1)    if(deblocking_filter_override_flag ) {    slice_deblocking_filter_disabled_flag u(1)     if(!slice_deblocking_filter_disabled_flag ) {      slice_beta_offset_div2se(v)      slice_tc_offset_div2 se(v)     }    }    if(pps_loop_filter_across_slices_enabled_flag &&     ( slice_sao_luma_flag|| slice_sao_chroma_flag ||      !slice_deblocking_filter_disabled_flag) )     slice_loop_filter_across_slices_enabled_flag u(1)  }  if(tiles_enabled_flag || entropy_coding_sync_enabled_flag ) {  num_entry_point_offsets ue(v)   if( num_entry_point_offsets > 0 ) {   offset_len_minus1 ue(v)    for( i = 0; i < num_entry_point_offsets;i++ )     entry_point_offset_minus1[ i ] u(v)   }  }  if(slice_segment_header_extension_present_flag ) {  slice_segment_header_extension_length ue(v)   for( i = 0; i <slice_segment_header_extension_length; i++)   slice_segment_header_extension_data_byte[ i ] u(8)  } byte_alignment( ) }

The syntax element num_extra_slice_header_bits is specified in the PPS.The slice_reserved_flag[i] is specified as follows in HEVC:slice_reserved_flag[i] has semantics and values that are reserved forfuture use by ITU-T|ISO/IEC. It can therefore be used in MV-HEVC/SHVC orother extensions to specify decoding operations that are associated withbase layer pictures.

In some embodiments, one of the slice_reserved_flag[i] flags is used asa triggering mechanism for layer-wise start-up process. For example, thefollowing syntax may be used:

slice_segment_header( ) { Descriptor  first_slice_segment_in_pic_flagu(1)  if( nal_unit_type >= BLA_W_LP && nal_unit_type <= RSV_IRAP_VCL23 )  no_output_of_prior_pics_flag u(1)  slice_pic_parameter_set_id ue(v) if( !first_slice_segment_in_pic_flag ) {   if(dependent_slice_segments_enabled_flag )    dependent_slice_segment_flagu(1)   slice_segment_address u(v)  }  if( !dependent_slice_segment_flag) {   if( num_extra_slice_header_bits > 0 ) {    extraBitsRead = 1   discardable_flag u(1)    if( num_extra_slice_header_bits > 1 ) {    extraBitsRead = 2     cross_layer_bla_flag u(1)    }    for( i =extraBitsRead;    i < num_extra_slice_header_bits ; i++ )    slice_reserved_flag[ i ] u(1)   }   slice_type ue(v)   ...

In some embodiments, the cross_layer_bla_flag may be present only inslice segment headers of certain NAL unit types only. In someembodiments, the cross_layer_bla_flag may be present only in slicesegment headers of certain layer(s) only, such as the base layer only oreach layer not depending on any other layer.

In some embodiments, when cross_layer_bla_flag is equal to 1, alayer-wise start-up process is used in decoding. Whencross_layer_bla_flag is equal to 0, a layer-wise start-up process is notused in decoding.

A decoding process may be specified in a manner that a certain variablecontrols whether or not a layer-wise start-up process is used. Forexample, a variable NoClrasOutputFlag may be used, which, when equal to0, indicates a normal decoding operation, and when equal to 1, indicatesa layer-wise start-up operation. NoClrasOutputFlag may be set forexample using one or more of the following steps:

-   -   1) If the current picture is an IRAP picture that is the first        picture in the bitstream, NoClrasOutputFlag is set equal to 1.    -   2) Otherwise, if some external means are available to set the        variable NoClrasOutputFlag equal to a value for a base-layer        IRAP picture, the variable NoClrasOutputFlag is set equal to the        value provided by the external means.    -   3) Otherwise, if the current picture is a BLA picture that is        the first picture in a coded video sequence (CVS),        NoClrasOutputFlag is set equal to 1.    -   4) Otherwise, if the current picture is an IDR picture that is        the first picture in a coded video sequence (CVS) and        cross_layer_bla_flag is equal to 1, NoClrasOutputFlag is set        equal to 1.    -   5) Otherwise, NoClrasOutputFlag is set equal to 0.

Step 4 above may alternatively be phrased more generally for example asfollows: Otherwise, if the current picture is an IRAP picture that isthe first picture in a CVS and an indication of layer-wise start-upprocess is associated with the IRAP picture, NoClrasOutputFlag is setequal to 1. It should be understood that other ways to phrase thecondition are possible and equally applicable.

A decoding process for layer-wise start-up may be for example controlledby two array variables LayerInitialisedFlag[i] andFirstPicInLayerDecodedFlag[i] which may have entries for each layer(possibly excluding the base layer and possibly other independent layerstoo). When the layer-wise start-up process is invoked, for example asresponse to NoClrasOutputFlag being equal to 1, these array variablesmay be reset to their default values. For example, when there are 64layers enabled (e.g. with a 6-bit nuh_layer_id), the variables may bereset as follows: the variable LayerInitialisedFlag[i] is set equal to 0for all values of i from 0 to 63, inclusive, and the variableFirstPicInLayerDecodedFlag[i] is set equal to 0 for all values of i from1 to 63, inclusive.

The decoding process may include the following or similar to control theoutput of RASL pictures. When the current picture is an IRAP picture,the following applies:

-   -   If LayerInitialisedFlag[nuh_layer_id] is equal to 0, the        variable NoRaslOutputFlag is set equal to 1.    -   Otherwise, if some external means is available to set the        variable HandleCraAsBlaFlag to a value for the current picture,        the variable HandleCraAsBlaFlag is set equal to the value        provided by the external means and the variable NoRaslOutputFlag        is set equal to HandleCraAsBlaFlag.    -   Otherwise, the variable HandleCraAsBlaFlag is set equal to 0 and        the variable NoRaslOutputFlag is set equal to 0.

In some embodiments, the decoding process may include the following toupdate the LayerInitialisedFlag for a layer. When the current picture isan IRAP picture and either one of the following is true,LayerInitialisedFlag[nuh_layer_id] is set equal to 1.

-   -   nuh_layer_id is equal to 0.    -   LayerInitialisedFlag[nuh_layer_id] is equal to 0 and        LayerInitialisedFlag[refLayerId] is equal to 1 for all values of        refLayerId equal to RefLayerId[nuh_layer_id][j], where j is in        the range of 0 to NumDirectRefLayers[nuh_layer_id]−1, inclusive.

In some embodiments, the encoder process may be constrained so that itis required that when the current picture is an IRAP picture, either oneof the following is true:

-   -   nuh_layer_id is equal to 0.    -   LayerInitialisedFlag[refLayerId] is equal to 1 for all values of        refLayerId equal to RefLayerId[nuh_layer_id][j], where j is in        the range of 0 to NumDirectRefLayers[nuh_layer_id]−1, inclusive.

In some embodiments, the above-mentioned constrain in the encodingprocess may be pre-defined for example in a coding standard. In someembodiments, the above-mentioned constraint may be indicated by theencoder in the bitstream. If the above-mentioned constraint ispre-defined or decoded by from the bitstream to be followed in abitstream or a part thereof, the decoding process may setLayerInitialisedFlag[nuh_layer_id] equal to 1 when the current pictureis an IRAP picture.

When FirstPicInLayerDecodedFlag[nuh_layer_id] is equal to 0, thedecoding process for generating unavailable reference pictures may beinvoked prior to decoding the current picture. The decoding process forgenerating unavailable reference pictures may generate pictures for eachpicture in a reference picture set with default values. The process ofgenerating unavailable reference pictures may be primarily specifiedonly for the specification of syntax constraints for CL-RAS pictures,where a CL-RAS picture may be defined as a picture with nuh_layer_idequal to layerId and LayerInitialisedFlag[layerId] is equal to 0. In HRDoperations, CL-RAS pictures may need to be taken into consideration inderivation of CPB arrival and removal times. In some embodiments,decoders may ignore any CL-RAS pictures, as these pictures are notspecified for output and have no effect on the decoding process of anyother pictures that are specified for output.

In some embodiments, the same syntax element to indicatecross_layer_bla_flag or similar may be used to indicate CL-RAS picturesor potential CL-RAS pictures. CL-RAS pictures may have the property thatwhen a layer-wise start-up mechanism is invoked (e.g. whenNoClrasOutputFlag is equal to 1), the CL-RAS pictures are not output andmay not be correctly decodable, as the CL-RAS picture may containreferences to pictures that are not present in the bitstream. It may bespecified that CL-RAS pictures are not used as reference pictures forthe decoding process of non-CL-RAS pictures.

An example embodiment of using the same syntax element to indicatecross_layer_bla_flag (or similar) and CL-RAS pictures is presented belowwith relation to the HEVC slice segment header syntax.

slice_segment_header( ) { Descriptor  first_slice_segment_in_pic_flagu(1)  if( nal_unit_type >= BLA_W_LP && nal_unit_type <= RSV_IRAP_VCL23 )  no_output_of_prior_pics_flag u(1)  slice_pic_parameter_set_id ue(v) if( !first_slice_segment_in_pic_flag ) {   if(dependent_slice_segments_enabled_flag )    dependent_slice_segment_flagu(1)   slice_segment_address u(v)  }  if( !dependent_slice_segment_flag) {   if( num_extra_slice_header_bits > 0 ) {    extraBitsRead = 1   discardable_flag u(1)    if( num_extra_slice_header_bits > 1 ) {    extraBitsRead = 2     cross_layer_constraint_flag u(1)    }    for(i = extraBitsRead;    i < num_extra_slice_header_bits ; i++ )    slice_reserved_flag[ i ] u(1)   }   slice_type ue(v)   ...

A picture may be considered as a CL-RAS picture when it is a non-IRAPpicture (e.g. as determined by its NAL unit type), it resides in anenhancement layer and it has cross_layer_constraint_flag (or similar)equal to 1. Otherwise, a picture may be classified of being a non-CL-RASpicture. cross_layer_bla_flag may be inferred to be equal to 1 (or arespective variable may be set to 1), if the picture is an IRAP picture(or in some embodiments, an IDR picture) (e.g. as determined by its NALunit type), it resides in the base layer, andcross_layer_constraint_flag is equal to 1. Otherwise,cross_layer_bla_flag may inferred to be equal to 0 (or a respectivevariable may be set to 0). Various embodiments may be applied withCL-RAS pictures identified like this and/or cross_layer_bla_flag (orsimilar) identified like this.

In some embodiments, a splicer inserts said indication(s) in thebitstream when concatenating a second coded video sequence after a firstcoded video sequence. A splicer may insert an indication ofcross_layer_bla_flag (or similar) or change the value ofcross_layer_bla_flag (or similar) and hence indicate triggering of alayer-wise start-up mechanism in decoders receiving the bitstream or atleast a part containing the second coded video sequence. A splicer mayadditionally insert indication(s) of CL-RAS pictures.

FIG. 4 shows a block diagram of a video encoder suitable for employingembodiments of the invention. FIG. 4 presents an encoder for two layers,but it would be appreciated that presented encoder could be similarlyextended to encode more than two layers. FIG. 4 illustrates anembodiment of a video encoder comprising a first encoder section 500 fora base layer and a second encoder section 502 for an enhancement layer.Each of the first encoder section 500 and the second encoder section 502may comprise similar elements for encoding incoming pictures. Theencoder sections 500, 502 may comprise a pixel predictor 302, 402,prediction error encoder 303, 403 and prediction error decoder 304, 404.FIG. 4 also shows an embodiment of the pixel predictor 302, 402 ascomprising an inter-predictor 306, 406, an intra-predictor 308, 408, amode selector 310, 410, a filter 316, 416, and a reference frame memory318, 418. The pixel predictor 302 of the first encoder section 500receives 300 base layer images of a video stream to be encoded at boththe inter-predictor 306 (which determines the difference between theimage and a motion compensated reference frame 318) and theintra-predictor 308 (which determines a prediction for an image blockbased only on the already processed parts of current frame or picture).The output of both the inter-predictor and the intra-predictor arepassed to the mode selector 310. The intra-predictor 308 may have morethan one intra-prediction modes. Hence, each mode may perform theintra-prediction and provide the predicted signal to the mode selector310. The mode selector 310 also receives a copy of the base layerpicture 300. Correspondingly, the pixel predictor 402 of the secondencoder section 502 receives 400 enhancement layer images of a videostream to be encoded at both the inter-predictor 406 (which determinesthe difference between the image and a motion compensated referenceframe 418) and the intra-predictor 408 (which determines a predictionfor an image block based only on the already processed parts of currentframe or picture). The output of both the inter-predictor and theintra-predictor are passed to the mode selector 410. The intra-predictor408 may have more than one intra-prediction modes. Hence, each mode mayperform the intra-prediction and provide the predicted signal to themode selector 410. The mode selector 410 also receives a copy of theenhancement layer picture 400.

The mode selector 310 may use, in the cost evaluator block 382, forexample Lagrangian cost functions to choose between coding modes andtheir parameter values, such as motion vectors, reference indexes, andintra prediction direction, typically on block basis. This kind of costfunction may use a weighting factor lambda to tie together the (exact orestimated) image distortion due to lossy coding methods and the (exactor estimated) amount of information that is required to represent thepixel values in an image area: C=D+lambda×R, where C is the Lagrangiancost to be minimized, D is the image distortion (e.g. Mean SquaredError) with the mode and their parameters, and R the number of bitsneeded to represent the required data to reconstruct the image block inthe decoder (e.g. including the amount of data to represent thecandidate motion vectors).

Depending on which encoding mode is selected to encode the currentblock, the output of the inter-predictor 306, 406 or the output of oneof the optional intra-predictor modes or the output of a surface encoderwithin the mode selector is passed to the output of the mode selector310, 410. The output of the mode selector is passed to a first summingdevice 321, 421. The first summing device may subtract the output of thepixel predictor 302, 402 from the base layer picture 300/enhancementlayer picture 400 to produce a first prediction error signal 320, 420which is input to the prediction error encoder 303, 403.

The pixel predictor 302, 402 further receives from a preliminaryreconstructor 339, 439 the combination of the prediction representationof the image block 312, 412 and the output 338, 438 of the predictionerror decoder 304, 404. The preliminary reconstructed image 314, 414 maybe passed to the intra-predictor 308, 408 and to a filter 316, 416. Thefilter 316, 416 receiving the preliminary representation may filter thepreliminary representation and output a final reconstructed image 340,440 which may be saved in a reference frame memory 318, 418. Thereference frame memory 318 may be connected to the inter-predictor 306to be used as the reference image against which a future base layerpictures 300 is compared in inter-prediction operations. Subject to thebase layer being selected and indicated to be source for inter-layersample prediction and/or inter-layer motion information prediction ofthe enhancement layer according to some embodiments, the reference framememory 318 may also be connected to the inter-predictor 406 to be usedas the reference image against which a future enhancement layer pictures400 is compared in inter-prediction operations. Moreover, the referenceframe memory 418 may be connected to the inter-predictor 406 to be usedas the reference image against which a future enhancement layer pictures400 is compared in inter-prediction operations.

Filtering parameters from the filter 316 of the first encoder section500 may be provided to the second encoder section 502 subject to thebase layer being selected and indicated to be source for predicting thefiltering parameters of the enhancement layer according to someembodiments.

The prediction error encoder 303, 403 comprises a transform unit 342,442 and a quantizer 344, 444. The transform unit 342, 442 transforms thefirst prediction error signal 320, 420 to a transform domain. Thetransform is, for example, the DCT transform. The quantizer 344, 444quantizes the transform domain signal, e.g. the DCT coefficients, toform quantized coefficients.

The prediction error decoder 304, 404 receives the output from theprediction error encoder 303, 403 and performs the opposite processes ofthe prediction error encoder 303, 403 to produce a decoded predictionerror signal 338, 438 which, when combined with the predictionrepresentation of the image block 312, 412 at the second summing device339, 439, produces the preliminary reconstructed image 314, 414. Theprediction error decoder may be considered to comprise a dequantizer361, 461, which dequantizes the quantized coefficient values, e.g. DCTcoefficients, to reconstruct the transform signal and an inversetransformation unit 363, 463, which performs the inverse transformationto the reconstructed transform signal wherein the output of the inversetransformation unit 363, 463 contains reconstructed block(s). Theprediction error decoder may also comprise a block filter which mayfilter the reconstructed block(s) according to further decodedinformation and filter parameters.

The entropy encoder 330, 430 receives the output of the prediction errorencoder 303, 403 and may perform a suitable entropy encoding/variablelength encoding on the signal to provide error detection and correctioncapability. The outputs of the entropy encoders 330, 430 may be insertedinto a bitstream e.g. by a multiplexer 508.

For completeness a suitable decoder is hereafter described. However,some decoders may not be able to process enhancement layer data whereinthey may not be able to decode all received images.

At the decoder side similar operations may be performed to reconstructthe image blocks. FIG. 5 shows a block diagram of a video decoder 550suitable for employing embodiments of the invention. In this embodimentthe video decoder 550 comprises a first decoder section 552 for baseview components and a second decoder section 554 for non-base viewcomponents. Block 556 illustrates a demultiplexer for deliveringinformation regarding base view components to the first decoder section552 and for delivering information regarding non-base view components tothe second decoder section 554. The decoder shows an entropy decoder700, 800 which performs an entropy decoding (E⁻¹) on the receivedsignal. The entropy decoder thus performs the inverse operation to theentropy encoder 330, 430 of the encoder described above. The entropydecoder 700, 800 outputs the results of the entropy decoding to aprediction error decoder 701, 801 and pixel predictor 704, 804.Reference P′_(n) stands for a predicted representation of an imageblock. Reference D′n stands for a reconstructed prediction error signal.Blocks 705, 805 illustrate preliminary reconstructed images or imageblocks (I′_(n)). Reference R′_(n) stands for a final reconstructed imageor image block. Blocks 703, 803 illustrate inverse transform (T⁻¹).Blocks 702, 802 illustrate inverse quantization (Q⁻¹). Blocks 706, 806illustrate a reference frame memory (RFM). Blocks 707, 807 illustrateprediction (P) (either inter prediction or intra prediction). Blocks708, 808 illustrate filtering (F). Blocks 709, 809 may be used tocombine decoded prediction error information with predicted baseview/non-base view components to obtain the preliminary reconstructedimages (I′_(n)). Preliminary reconstructed and filtered base view imagesmay be output 710 from the first decoder section 552 and preliminaryreconstructed and filtered base view images may be output 810 from thesecond decoder section 554.

The pixel predictor 704, 804 receives the output of the entropy decoder700, 800. The output of the entropy decoder 700, 800 may include anindication on the prediction mode used in encoding the current block. Apredictor selector 707, 807 within the pixel predictor 704, 804 maydetermine that the current block to be decoded is an enhancement layerblock. Hence, the predictor selector 707, 807 may select to useinformation from a corresponding block on another layer such as the baselayer to filter the base layer prediction block while decoding thecurrent enhancement layer block. An indication that the base layerprediction block has been filtered before using in the enhancement layerprediction by the encoder may have been received by the decoder whereinthe pixel predictor 704, 804 may use the indication to provide thereconstructed base layer block values to the filter 708, 808 and todetermine which kind of filter has been used, e.g. the SAO filter and/orthe adaptive loop filter, or there may be other ways to determinewhether or not the modified decoding mode should be used.

The predictor selector may output a predicted representation of an imageblock P′_(n) to a first combiner 709. The predicted representation ofthe image block is used in conjunction with the reconstructed predictionerror signal D′n to generate a preliminary reconstructed image I′_(n).The preliminary reconstructed image may be used in the predictor 704,804 or may be passed to a filter 708, 808. The filter applies afiltering which outputs a final reconstructed signal R′_(n). The finalreconstructed signal R′_(n) may be stored in a reference frame memory706, 806, the reference frame memory 706, 806 further being connected tothe predictor 707, 807 for prediction operations.

The prediction error decoder 702, 802 receives the output of the entropydecoder 700, 800. A dequantizer 702, 802 of the prediction error decoder702, 802 may dequantize the output of the entropy decoder 700, 800 andthe inverse transform block 703, 803 may perform an inverse transformoperation to the dequantized signal output by the dequantizer 702, 802.The output of the entropy decoder 700, 800 may also indicate thatprediction error signal is not to be applied and in this case theprediction error decoder produces an all zero output signal.

It should be understood that for various blocks in FIG. 5 inter-layerprediction may be applied, even if it is not illustrated in FIG. 5.Inter-layer prediction may include sample prediction and/orsyntax/parameter prediction. For example, a reference picture from onedecoder section (e.g. RFM 706) may be used for sample prediction of theother decoder section (e.g. block 807). In another example, syntaxelements or parameters from one decoder section (e.g. filter parametersfrom block 708) may be used for syntax/parameter prediction of the otherdecoder section (e.g. block 808).

In some embodiments the views may be coded with another standard otherthan H.264/AVC or HEVC.

FIG. 1 shows a block diagram of a video coding system according to anexample embodiment as a schematic block diagram of an exemplaryapparatus or electronic device 50, which may incorporate a codecaccording to an embodiment of the invention. FIG. 2 shows a layout of anapparatus according to an example embodiment. The elements of FIGS. 1and 2 will be explained next.

The electronic device 50 may for example be a mobile terminal or userequipment of a wireless communication system. However, it would beappreciated that embodiments of the invention may be implemented withinany electronic device or apparatus which may require encoding anddecoding or encoding or decoding video images.

The apparatus 50 may comprise a housing 30 for incorporating andprotecting the device. The apparatus 50 further may comprise a display32 in the form of a liquid crystal display. In other embodiments of theinvention the display may be any suitable display technology suitable todisplay an image or video. The apparatus 50 may further comprise akeypad 34. In other embodiments of the invention any suitable data oruser interface mechanism may be employed. For example the user interfacemay be implemented as a virtual keyboard or data entry system as part ofa touch-sensitive display. The apparatus may comprise a microphone 36 orany suitable audio input which may be a digital or analogue signalinput. The apparatus 50 may further comprise an audio output devicewhich in embodiments of the invention may be any one of: an earpiece 38,speaker, or an analogue audio or digital audio output connection. Theapparatus 50 may also comprise a battery 40 (or in other embodiments ofthe invention the device may be powered by any suitable mobile energydevice such as solar cell, fuel cell or clockwork generator). Theapparatus may further comprise a camera 42 capable of recording orcapturing images and/or video. In some embodiments the apparatus 50 mayfurther comprise an infrared port for short range line of sightcommunication to other devices. In other embodiments the apparatus 50may further comprise any suitable short range communication solutionsuch as for example a Bluetooth wireless connection or a USB/firewirewired connection.

The apparatus 50 may comprise a controller 56 or processor forcontrolling the apparatus 50. The controller 56 may be connected tomemory 58 which in embodiments of the invention may store both data inthe form of image and audio data and/or may also store instructions forimplementation on the controller 56. The controller 56 may further beconnected to codec circuitry 54 suitable for carrying out coding anddecoding of audio and/or video data or assisting in coding and decodingcarried out by the controller 56.

The apparatus 50 may further comprise a card reader 48 and a smart card46, for example a UICC and UICC reader for providing user informationand being suitable for providing authentication information forauthentication and authorization of the user at a network.

The apparatus 50 may comprise radio interface circuitry 52 connected tothe controller and suitable for generating wireless communicationsignals for example for communication with a cellular communicationsnetwork, a wireless communications system or a wireless local areanetwork. The apparatus 50 may further comprise an antenna 44 connectedto the radio interface circuitry 52 for transmitting radio frequencysignals generated at the radio interface circuitry 52 to otherapparatus(es) and for receiving radio frequency signals from otherapparatus(es).

In some embodiments of the invention, the apparatus 50 comprises acamera capable of recording or detecting individual frames which arethen passed to the codec 54 or controller for processing. In someembodiments of the invention, the apparatus may receive the video imagedata for processing from another device prior to transmission and/orstorage. In some embodiments of the invention, the apparatus 50 mayreceive either wirelessly or by a wired connection the image forcoding/decoding.

FIG. 3 shows an arrangement for video coding comprising a plurality ofapparatuses, networks and network elements according to an exampleembodiment. With respect to FIG. 3, an example of a system within whichembodiments of the present invention can be utilized is shown. Thesystem 10 comprises multiple communication devices which can communicatethrough one or more networks. The system 10 may comprise any combinationof wired or wireless networks including, but not limited to a wirelesscellular telephone network (such as a GSM, UMTS, CDMA network etc), awireless local area network (WLAN) such as defined by any of the IEEE802.x standards, a Bluetooth personal area network, an Ethernet localarea network, a token ring local area network, a wide area network, andthe Internet.

The system 10 may include both wired and wireless communication devicesor apparatus 50 suitable for implementing embodiments of the invention.For example, the system shown in FIG. 3 shows a mobile telephone network11 and a representation of the internet 28. Connectivity to the internet28 may include, but is not limited to, long range wireless connections,short range wireless connections, and various wired connectionsincluding, but not limited to, telephone lines, cable lines, powerlines, and similar communication pathways.

The example communication devices shown in the system 10 may include,but are not limited to, an electronic device or apparatus 50, acombination of a personal digital assistant (PDA) and a mobile telephone14, a PDA 16, an integrated messaging device (IMD) 18, a desktopcomputer 20, a notebook computer 22. The apparatus 50 may be stationaryor mobile when carried by an individual who is moving. The apparatus 50may also be located in a mode of transport including, but not limitedto, a car, a truck, a taxi, a bus, a train, a boat, an airplane, abicycle, a motorcycle or any similar suitable mode of transport.

Some or further apparatuses may send and receive calls and messages andcommunicate with service providers through a wireless connection 25 to abase station 24. The base station 24 may be connected to a networkserver 26 that allows communication between the mobile telephone network11 and the internet 28. The system may include additional communicationdevices and communication devices of various types.

The communication devices may communicate using various transmissiontechnologies including, but not limited to, code division multipleaccess (CDMA), global systems for mobile communications (GSM), universalmobile telecommunications system (UMTS), time divisional multiple access(TDMA), frequency division multiple access (FDMA), transmission controlprotocol-internet protocol (TCP-IP), short messaging service (SMS),multimedia messaging service (MMS), email, instant messaging service(IMS), Bluetooth, IEEE 802.11 and any similar wireless communicationtechnology. A communications device involved in implementing variousembodiments of the present invention may communicate using various mediaincluding, but not limited to, radio, infrared, laser, cableconnections, and any suitable connection.

In the above, some embodiments have been described in relation toparticular types of parameter sets. It needs to be understood, however,that embodiments could be realized with any type of parameter set orother syntax structure in the bitstream.

In the above, some embodiments have been described in relation toencoding indications, syntax elements, and/or syntax structures into abitstream or into a coded video sequence and/or decoding indications,syntax elements, and/or syntax structures from a bitstream or from acoded video sequence. It needs to be understood, however, thatembodiments could be realized when encoding indications, syntaxelements, and/or syntax structures into a syntax structure or a dataunit that is external from a bitstream or a coded video sequencecomprising video coding layer data, such as coded slices, and/ordecoding indications, syntax elements, and/or syntax structures from asyntax structure or a data unit that is external from a bitstream or acoded video sequence comprising video coding layer data, such as codedslices.

In the above, the example embodiments have been described with the helpof syntax of the bitstream. It needs to be understood, however, that thecorresponding structure and/or computer program may reside at theencoder for generating the bitstream and/or at the decoder for decodingthe bitstream. Likewise, where the example embodiments have beendescribed with reference to an encoder, it needs to be understood thatthe resulting bitstream and the decoder have corresponding elements inthem. Likewise, where the example embodiments have been described withreference to a decoder, it needs to be understood that the encoder hasstructure and/or computer program for generating the bitstream to bedecoded by the decoder. Likewise, where example embodiments have beendescribed with reference to a splicer, it needs to be understood that asplicer could likewise be an encoder, a middle-box, or any other entitythat creates or modifies a coded video bitstream.

In the above, some embodiments have been described with reference to anenhancement layer and a reference layer, where the reference layer maybe for example a base layer.

In the above, some embodiments have been described with reference to anenhancement view and a reference view, where the reference view may befor example a base view.

In the above, some embodiments have been described with reference tomotion information prediction. It needs to be understood thatembodiments could be realized by applying motion information inheritancerather than motion information prediction.

In the above, methods have been described with reference to a block orblocks, where the blocks may be selected in various ways. For example,the block may be a unit for motion prediction, i.e. a block that has itsown motion information associated with it, such as a prediction unit(PU) in HEVC. In another example, the block may be a unit for storingmotion information for a decoded reference picture. Methods may berealized with different selection of the unit for a block.

It needs to be understood that embodiments may be applicable to anytypes of layered coding, for example for multiview coding, qualityscalability, spatial scalability, and for multiview video plus depthcoding.

Embodiments of the present invention may be implemented in software,hardware, application logic or a combination of software, hardware andapplication logic. In an example embodiment, the application logic,software or an instruction set is maintained on any one of variousconventional computer-readable media. In the context of this document, a“computer-readable medium” may be any media or means that can contain,store, communicate, propagate or transport the instructions for use byor in connection with an instruction execution system, apparatus, ordevice, such as a computer, with one example of a computer described anddepicted in FIGS. 1 and 2. A computer-readable medium may comprise acomputer-readable storage medium that may be any media or means that cancontain or store the instructions for use by or in connection with aninstruction execution system, apparatus, or device, such as a computer.

If desired, the different functions discussed herein may be performed ina different order and/or concurrently with each other. Furthermore, ifdesired, one or more of the above-described functions may be optional ormay be combined.

Although the above examples describe embodiments of the inventionoperating within a codec within an electronic device, it would beappreciated that the invention as described below may be implemented aspart of any video codec. Thus, for example, embodiments of the inventionmay be implemented in a video codec which may implement video codingover fixed or wired communication paths.

Thus, user equipment may comprise a video codec such as those describedin embodiments of the invention above. It shall be appreciated that theterm user equipment is intended to cover any suitable type of wirelessuser equipment, such as mobile telephones, portable data processingdevices or portable web browsers.

Furthermore elements of a public land mobile network (PLMN) may alsocomprise video codecs as described above.

In general, the various embodiments of the invention may be implementedin hardware or special purpose circuits, software, logic or anycombination thereof. For example, some aspects may be implemented inhardware, while other aspects may be implemented in firmware or softwarewhich may be executed by a controller, microprocessor or other computingdevice, although the invention is not limited thereto. While variousaspects of the invention may be illustrated and described as blockdiagrams, flow charts, or using some other pictorial representation, itis well understood that these blocks, apparatuses, systems, techniquesor methods described herein may be implemented in, as non-limitingexamples, hardware, software, firmware, special purpose circuits orlogic, general purpose hardware or controller or other computingdevices, or some combination thereof.

The embodiments of this invention may be implemented by computersoftware executable by a data processor of the mobile device, such as inthe processor entity, or by hardware, or by a combination of softwareand hardware. Further in this regard it should be noted that any blocksof the logic flow as in the Figures may represent program steps, orinterconnected logic circuits, blocks and functions, or a combination ofprogram steps and logic circuits, blocks and functions. The software maybe stored on such physical media as memory chips, or memory blocksimplemented within the processor, magnetic media such as hard disk orfloppy disks, and optical media such as for example DVD and the datavariants thereof, CD.

The various embodiments of the invention can be implemented with thehelp of computer program code that resides in a memory and causes therelevant apparatuses to carry out the invention. For example, a terminaldevice may comprise circuitry and electronics for handling, receivingand transmitting data, computer program code in a memory, and aprocessor that, when running the computer program code, causes theterminal device to carry out the features of an embodiment. Yet further,a network device may comprise circuitry and electronics for handling,receiving and transmitting data, computer program code in a memory, anda processor that, when running the computer program code, causes thenetwork device to carry out the features of an embodiment.

The memory may be of any type suitable to the local technicalenvironment and may be implemented using any suitable data storagetechnology, such as semiconductor-based memory devices, magnetic memorydevices and systems, optical memory devices and systems, fixed memoryand removable memory. The data processors may be of any type suitable tothe local technical environment, and may include one or more of generalpurpose computers, special purpose computers, microprocessors, digitalsignal processors (DSPs) and processors based on multi-core processorarchitecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various componentssuch as integrated circuit modules. The design of integrated circuits isby and large a highly automated process. Complex and powerful softwaretools are available for converting a logic level design into asemiconductor circuit design ready to be etched and formed on asemiconductor substrate.

Programs, such as those provided by Synopsys Inc., of Mountain View,Calif. and Cadence Design, of San Jose, Calif. automatically routeconductors and locate components on a semiconductor chip using wellestablished rules of design as well as libraries of pre-stored designmodules. Once the design for a semiconductor circuit has been completed,the resultant design, in a standardized electronic format (e.g., Opus,GDSII, or the like) may be transmitted to a semiconductor fabricationfacility or “fab” for fabrication.

The foregoing description has provided by way of exemplary andnon-limiting examples a full and informative description of theexemplary embodiment of this invention. However, various modificationsand adaptations may become apparent to those skilled in the relevantarts in view of the foregoing description, when read in conjunction withthe accompanying drawings and the appended claims. However, all such andsimilar modifications of the teachings of this invention will still fallwithin the scope of this invention.

In the following some examples will be provided.

According to a first example, there is provided a method comprising:

-   -   receiving or obtaining a bitstream including coded pictures in        two or more scalability layers;    -   identifying a first coded picture within the bitstream, the        first coded picture having a picture type that may be used to        start decoding;    -   determining that a layer-wise start-up is initiated with the        first coded picture; and    -   indicating in the bitstream an association of the first coded        picture with a layer-wise start-up.

According to an embodiment, the method further comprises:

-   -   indicating in the bitstream said association separately from        indicating a type of the first coded picture.

According to a second example, there is provided a method comprising:

-   -   receiving or obtaining a bitstream including coded pictures in        two or more scalability layers;    -   identifying a first coded picture within the bitstream, the        first coded picture having a picture type that may be used to        start decoding;    -   determining that a layer-wise start-up is initiated with the        first coded picture on the basis of an indication received in or        along the bitstream of an association of the first coded picture        with a layer-wise start-up.

According to an embodiment, the method further comprises:

-   -   wherein the indication of said association is separate from an        indication of a type of the first coded picture.

According to a third example, there is provided an apparatus comprisingmeans for:

-   -   receiving or obtaining a bitstream including coded pictures in        two or more scalability layers;    -   identifying a first coded picture within the bitstream, the        first coded picture having a picture type that may be used to        start decoding;    -   determining that a layer-wise start-up is initiated with the        first coded picture; and    -   indicating in the bitstream an association of the first coded        picture with a layer-wise start-up.

According to an embodiment, the apparatus further comprises means for:

-   -   indicating in the bitstream said association separately from        indicating a type of the first coded picture.

We claim:
 1. An apparatus comprising at least one processor and at leastone memory, said at least one memory stored with code thereon, whichwhen executed by said at least one processor, causes an apparatus toperform at least the following: receive a bitstream including codedpictures in two or more scalability layers or encoding pictures into abitstream in two or more scalability layers; identify a first codedpicture within the bitstream, the first coded picture having a picturetype that may be used to start decoding; determine that a layer-wisestart-up is initiated with the first coded picture; indicate in or alongthe bitstream an association of the first coded picture with thelayer-wise start-up, wherein the layer-wise start-up comprises:controlling the layer-wise startup by a first array variable, whereineach scalability layer of index i is associated with an entry in thefirst array variable indicative of whether correct decoding of thescalability layer corresponding to the entry in the first array variablehas been initialized; and controlling the layer-wise startup by a secondarray variable, wherein each scalability layer of index i is associatedwith an entry in the second array variable indicative of whether apicture in the scalability layer corresponding to the entry in thesecond array variable has been decoded but may have been decodedincorrectly; and cause one or more decoded pictures to be output.
 2. Theapparatus according to claim 1 wherein the apparatus is further causedto, when the layer-wise start-up is initiated, set the first arrayvariable equal to 0 and the second array variable equal to 0 for allscalability layers other than the scalability layer with index i equalto
 0. 3. The apparatus according to claim 1 wherein the apparatus isfurther caused to, when the second array variable of the scalabilitylayer with index i is equal to 0, generate unavailable pictures for thereference pictures of the first picture in decoding order in anenhancement layer with index i.
 4. The apparatus according to claim 1,wherein the apparatus is further caused to indicate in the bitstream atype of the first coded picture separately from indicating saidassociation.
 5. An apparatus comprising at least one processor and atleast one memory, said at least one memory stored with code thereon,which when executed by said at least one processor, causes an apparatusto perform at least the following: receive or obtain a bitstreamincluding coded pictures in two or more scalability layers; identify afirst coded picture within the bitstream, the first coded picture havinga picture type that may be used to start decoding; conclude that alayer-wise start-up is initiated with the first coded picture on a basisof an indication received in or along the bitstream of an association ofthe first coded picture with the layer-wise start-up, wherein thelayer-wise start-up comprises: controlling the layer-wise startup by afirst array variable, wherein each scalability layer of index i isassociated with an entry in the first array variable indicative ofwhether correct decoding of the scalability layer corresponding to theentry in the first array variable has been initialized; and controllingthe layer-wise startup by a second array variable, wherein eachscalability layer of index i is associated with an entry in the secondarray variable indicative of whether a picture in the scalability layercorresponding to the entry in the second array variable has been decodedbut may have been decoded incorrectly; and cause one or more decodedpictures to be output.
 6. The apparatus according to claim 5 wherein theapparatus is further caused to, when the layer-wise start-up isinitiated, set the first array variable equal to 0 and the second arrayvariable equal to 0 for all scalability layers other than thescalability layer with index i equal to
 0. 7. The apparatus according toclaim 5 wherein the apparatus is further caused to, when the secondarray variable of the scalability layer with index i is equal to 0,generate unavailable pictures for the reference pictures of the firstpicture in decoding order in an enhancement layer with index i.
 8. Theapparatus according to claim 5 wherein the apparatus is further causedto receive an indication of a type of the first coded picture separatefrom said indication of the association.
 9. A method comprising:receiving a bitstream including coded pictures in two or morescalability layers or encoding pictures into a bitstream in two or morescalability layers; identifying a first coded picture within thebitstream, the first coded picture having a picture type that may beused to start decoding; determining that a layer-wise start-up isinitiated with the first coded picture; indicating in or along thebitstream an association of the first coded picture with the layer-wisestart-up, wherein the layer-wise start-up comprises: controlling thelayer-wise startup by a first array variable, wherein each scalabilitylayer of index i is associated with an entry in the first array variableindicative of whether correct decoding of the scalability layercorresponding to the entry in the first array variable has beeninitialized; and controlling the layer-wise startup by a second arrayvariable, wherein each scalability layer of index i is associated withan entry in the second array variable indicative of whether a picture inthe scalability layer corresponding to the entry in the second arrayvariable has been decoded but may have been decoded incorrectly; andcausing one or more decoded pictures to be output.
 10. The methodaccording to claim 9 further comprising, when the layer-wise start-up isinitiated, setting the first array variable equal to 0 and the secondarray variable equal to 0 for all scalability layers other than thescalability layer with index i equal to
 0. 11. The method according toclaim 9 further comprising, when the second array variable of thescalability layer with index i is equal to 0, generating unavailablepictures for the reference pictures of the first picture in decodingorder in an enhancement layer with index i.
 12. The method according toclaim 9 wherein indicating in or along the bitstream said association isprovided in one or more of the following syntax structures: sliceheader, slice segment header; picture parameter set; group of slicesparameter set; picture header; access unit delimiter; picture delimiter;prefix network abstraction layer unit or suffix network abstractionlayer unit.
 13. The method according to claim 9 further comprisingindicating in the bitstream a type of the first coded picture separatelyfrom indicating said association.
 14. The method according to claim 9further comprising indicating said association in a 1-bit syntax elementin a slice header.
 15. The method according to claim 9 furthercomprising indicating said association in a 1-bit syntax element in aslice header wherein the syntax element is omitted in a single-layerdecoding process.
 16. The method according to claim 9 wherein thelayer-wise start-up further comprises one or more of the following:generating unavailable pictures for the reference pictures of the firstpicture in decoding order in the enhancement layer; omitting decoding ofa coded picture in the enhancement layer preceding an intra randomaccess point picture in the enhancement layer; starting an output ofpictures of the enhancement layer from an intra random access pointpicture in the enhancement layer when correct decoding of all referencelayers of the enhancement layer has been initialized; or omitting theoutput of random access skipped leading pictures associated with theintra random access point picture in the enhancement layer.
 17. Themethod according to claim 9 wherein the first array variable isLayerinitialisedFlag and the second array variable isFirstPiclnLayerDecodedFlag and index i identifies the scalability layerand wherein the method further comprises: when the layer-wise start-upis initiated, setting FirstPiclnLayerDecoded equal to 0 andLayerinitialisedFlag equal to 0 for all scalability layers other thanthe scalability layer with index i equal to 0; whenFirstPiclnLayerDecoded is equal to 0, generating unavailable picturesfor the reference pictures of the first picture in decoding order in anenhancement layer with index i; setting LayerinitialisedFlag equal to 1,when an intra random access point picture in an enhancement layer withindex i is decoded and LayerinitialisedFlag is equal to 1 for all valuesof j indicating the reference layers of the enhancement layer with indexi; and omitting an output of random access skipped leading picturesassociated with the intra random access point picture in the enhancementlayer for which LayerinitialisedFlag was set equal to
 1. 18. A methodcomprising: receiving or obtaining a bitstream including coded picturesin two or more scalability layers; identifying a first coded picturewithin the bitstream, the first coded picture having a picture type thatmay be used to start decoding; concluding that a layer-wise start-up isinitiated with the first coded picture on a basis of an indicationreceived in or along the bitstream of an association of the first codedpicture with the layer-wise start-up, wherein the layer-wise start-upcomprises: controlling the layer-wise startup by a first array variable,wherein each scalability layer of index i is associated with an entry inthe first array variable indicative of whether correct decoding of thescalability layer corresponding to the entry in the first array variablehas been initialized; and controlling the layer-wise startup by a secondarray variable, wherein each scalability layer of index i is associatedwith an entry in the second array variable indicative of whether apicture in the scalability layer corresponding to the entry in thesecond array variable has been decoded but may have been decodedincorrectly; and causing one or more decoded pictures to be output. 19.The method according to claim 18 further comprising, when the layer-wisestart-up is initiated, setting the first array variable equal to 0 andthe second array variable equal to 0 for all scalability layers otherthan the scalability layer with index i equal to
 0. 20. The methodaccording to claim 18 further comprising, when the second array variableof the scalability layer with index i is equal to 0, generatingunavailable pictures for the reference pictures of the first picture indecoding order in an enhancement layer with index i.
 21. The methodaccording to claim 18 wherein the indication is received in one or moreof the following syntax structures: slice header, slice segment header;picture parameter set; group of slices parameter set; picture header;access unit delimiter; picture delimiter; prefix network abstractionlayer unit or suffix network abstraction layer unit.
 22. The methodaccording to claim 18 further comprising receiving an indication of atype of the first coded picture separate from said indication of theassociation.
 23. The method according to claim 18 further comprisingreceiving the indication of the association in a 1-bit syntax element ina slice header.
 24. The method according to claim 18 further comprisingreceiving the indication of the association in a 1-bit syntax element ina slice header wherein the syntax element is omitted in a single-layerdecoding process.
 25. The method according to claim 18 wherein thelayer-wise start-up comprises one or more of the following: generatingunavailable pictures for the reference pictures of the first picture indecoding order in the enhancement layer; omitting decoding of a codedpicture in the enhancement layer preceding an intra random access pointpicture in the enhancement layer; starting an output of pictures of theenhancement layer from the intra random access point picture in theenhancement layer when correct decoding of all reference layers of theenhancement layer has been initialized; or omitting the output of randomaccess skipped leading pictures associated with the intra random accesspoint picture in the enhancement layer.
 26. The method according toclaim 18 wherein the first array variable is LayerinitialisedFlag andthe second array variable is FirstPiclnLayerDecodedFlag and index iidentifies the scalability layer and wherein the method furthercomprises the following: when the layer-wise start-up is initiated,setting FirstPiclnLayerDecoded equal to 0 and LayerlnitialisedFlag equalto 0 for all scalability layers other than the scalability layer withindex i equal to 0; when FirstPiclnLayerDecoded is equal to 0,generating unavailable pictures for the reference pictures of the firstpicture in decoding order in an enhancement layer with index i; settingLayerinitialisedFlag equal to 1, when an intra random access pointpicture in an enhancement layer with index i is decoded andLayerinitialisedFlag is equal to 1 for all values of j indicatingreference layers of the enhancement layer with index i; and omitting anoutput of random access skipped leading pictures associated with theintra random access point picture in the enhancement layer for whichLayerinitialisedFlag was set equal to
 1. 27. A computer program productembodied on a non-transitory computer readable medium, comprisingcomputer program code configured to, when executed on at least oneprocessor, cause an apparatus or a system to: receive a bitstreamincluding coded pictures in two or more scalability layers or encodingpictures into a bitstream in two or more scalability layers; identify afirst coded picture within the bitstream, the first coded picture havinga picture type that may be used to start decoding; determine that alayer-wise start-up is initiated with the first coded picture; indicatein or along the bitstream an association of the first coded picture withthe layer-wise start-up, wherein the layer-wise start-up comprises:controlling the layer-wise startup by a first array variable, whereineach scalability layer of index i is associated with an entry in thefirst array variable indicative of whether correct decoding of thescalability layer corresponding to the entry in the first array variablehas been initialized; and controlling the layer-wise startup by a secondarray variable, wherein each scalability layer of index i is associatedwith an entry in the second array variable indicative of whether apicture in the scalability layer corresponding to the entry in thesecond array variable has been decoded but may have been decodedincorrectly; and cause one or more decoded pictures to be output.
 28. Acomputer program product embodied on a non-transitory computer readablemedium, comprising computer program code configured to, when executed onat least one processor, cause an apparatus or a system to: receive orobtain a bitstream including coded pictures in two or more scalabilitylayers; identify a first coded picture within the bitstream, the firstcoded picture having a picture type that may be used to start decoding;conclude that a layer-wise start-up is initiated with the first codedpicture on a basis of an indication received in or along the bitstreamof an association of the first coded picture with the layer-wisestart-up, wherein the layer-wise start-up comprises: controlling thelayer-wise startup by a first array variable, wherein each scalabilitylayer of index i is associated with an entry in the first array variableindicative of whether correct decoding of the scalability layercorresponding to the entry in the first array variable has beeninitialized; and controlling the layer-wise startup by a second arrayvariable, wherein each scalability layer of index i is associated withan entry in the second array variable indicative of whether a picture inthe scalability layer corresponding to the entry in the second arrayvariable has been decoded but may have been decoded incorrectly; andcause one or more decoded pictures to be output.